■■■■ K t KHiilK? » I- 22502583442 Med K9768 Louis debacq, l’^macjejj de l«« ciasse ANIMAL PHYSICS. Digitized by the Internet Archive in 2016 https://archive.org/details/b28073708 7 NERVES OF THE FACE. _ /(ju /ki t ANIMAL PHYSICS; OR, THE BODY AND ITS FUNCTIONS, FAMILIARLY EXPLAINED By DIONYSIUS LARDNER, D.C.L., Formerly Professor of Natural Philosophy and Astronomy, in University College, London. WITH FIVE HUNDRED AND TWENTY ILLUSTRATIONS. LONDON : WALTON AND MABERLY, UPPER GOWER STREET AND IVY LANE, PATERNOSTER ROW. 1857. n %53 [. The Right of Translation is Reserved.'] WELLCOME INSTITUTE LIBRARY Coll. weiMQmec Call a* UQr ar ... . 1 LONDON : BRADBURY AND EVANS, PRINTERS, WHITEFRIARs. In the composition of this volume the Author has attempted to render a department of natural science, hitherto exclusively confined to the medical profession, accessible to all persons of ordinary education. The several subjects, to the exposition of which the volume is devoted, have, accordingly, been treated so as to be suited to readers of either sex and of any age. Since such a work, proceeding from the pen of one not of the medical profession, might be supposed to be liable to anatomical or physiological inaccuracies, the Author has induced several professors who feel an interest in the popular diffusion of tins branch of science, to read the sheets before being finally sent to press, and has gratefully availed himself of many suggestions arising from such revision. TABLE OF CONTENTS. CHAPTER I. GENERAL VIEW OF THE ANIMAL ORGANISATION. Par. Page I. Object of the work, and classes to whom it is addressed . . I 2. Structure of the body 3 Fluid constituents . . . 4 4- Skeleton ib. 5-8. Mechanism of the skeleton . ib. 9- Joints 5 IO. Defects incidental to them 6 11. Analogy to mechanical con- trivances . . . . ib. 12. Interosseous cartilage . . 7 >3- Friction of joints . 8 14- Adhesion of joints . . . 9 >5- Synovia, its uses ib. 16. Muscles 10 17- Tendons ; . . . . ii 1 8. Origin and insertion of muscles ib. J9- Their contractile power . . ib. 20. Antagonistic and cougenerate muscles 12 21. Muscular force . . . . 13 22. Involuntary muscles 14 IJ- Blood ib. 24. Lymph and chyle . 15 25-6. Circulation ib. 27- Respiration . 16 28. Source of nutrition . . . ib. *9- Nerves ib. 30. Neurilemma . . . . *7 3>- Brain ib. 31- Galvani's experiments . . 18 35- Aldini’saudUre 8 experiments ib. 34- Nerves of motion and sensa- tion *9 35. Digestive apparatus . . . 10 CHAPTER II. THE RONES AND LIGAMENTS. 37- Trunk .... . 21 38. Number of the bones. , . ib. Par. Tage 39- Growth of the bones 12 40. Constituents of the bones . . ib. 41. Cartilaginous skeletons . 2-3 42. Bones vary with age . . . ib. +3- Local adaptation of bones 24 44- Their analysis . . . . ib. 45- Nelaton's experiments 25 46. Section of bones . . . . ib. 47- Form of bones .... 26 48. The median plane . . . ib. 49- Long bones .... ib. 5°. Short bones . . . . 27 5>- Tabular bones . . . . ib. 52- Hair ib. 53- Distribution of the tissues of bones 28 54- Section of the ulna . ib. 55- Periosteum . . . . 30 S6. Structure of bones adapted to the forces acting upon them . ib. 57-9- Mechanical application of the muscles 31 60. Tubercles, processes, and con- dyles 32 61. Foramina, fissures, fossae, &c. ib. 62. Articulations . . . . 33 63. Examples of immovable joints ib. 64. The skull 34 65. The sutures 37 66. Mechanical principles of the cranium 38 67. Component tissues of its bones 39 68. Architectural analogies . . 40 69. Facial bones .... 4i 70. The under jaw . . . . ib. 71- Teeth ib. 72- Chin 44 73- Mechanism of lower jaw 45 74- Stylo-maxillary ligament . . 46 75- Muscles of lower jaw ib. 76. The vertebral column . . 47 77- Its structure .... 48 78. Intervertebral foramina . . 49 79- Provisions lor flexibility and elasticity . . . . ib. 80. Connection of the vertebrae . 5° 81. Movement of the spine . 51 82. Form and structure of the ver- tebral 52 83. General view of the spinal column 53 X TABLE OF CONTENTS. Par- _ . Page 84. Classification of the vertebras — spinal, dorsal, and lumbar. 53 8s. Number of vertebras . . 54 86. Sacrum 55 87. Ossa innomiuata . . . ib. 88. Intervertebral cartilages . . 56 89. Anterior and posterior com- mon ligaments . . .58 90. Articulation of the neck . . 59 91. The atlas ib. 9Z. The axis 60 93. The base of the skull . . . ib. 94. Its connection with the verte- bral column . . . .61 95. Thorax ib. 96. Scapulae and clavicles . . 63 97. The arm 64 98. The fore-arm . . . .66 99. The wrist ib. 100. Admirable mechanism of the arm 69 101. The lower member . . . 70 102. Hip joint ib. 103. The leg 72 104. The foot ib. ioj. Mechanism of ankle and foot . 76 CHAPTER III. THE MUSCLES. 106. Magnitude of the muscles . . 78 107. Their strength developed by exercise . . . . ib. 108. Examples in horses . . . 79 109. Tension of muscles . . 80 no. Their contractile force . . ib. in. Their statical force . . . 81 112. Their conditions of repose . . 82 113. Beneficial effects of intermit- ting tension .... ib. 114. Muscular contraction not uni- form ib. 115. Muscular sounds . . .83 116. Number of muscles . . . ib. 1 17. Classification . . . .84 118. Nomenclature . . . . ib. 119. Symmetry of muscularsystem ib. 120. Muscles act on the softer parts as well as on the bones . ib. 121. Development of muscles . . 85 122. Their distribution . . . ' 86 123. Their surprising subjection to the will ib. 124. Muscles of the face . . . ib. 125. Ditto of the trunk . . . 89 126. Their superposition in layers . ib. 127. Anterior and shouldermusclcs 93 128. Brachial muscles . . -94 129. Muscles on front of humerus 97 130. Ditto of fore-arm . . . 99 1 31. The hand . 105 132. Muscles of the lower member 107 133. Ditto of the hip . . . 108 134. Ditto of the leg . . . 109 135. Ditto of the foot. . . Ill CHAPTER IV. STRUCTURE OF THE LOWER A MM A Par. Pfcfe 136. Relation of human structure to that of lower animals *»4 >37- Facial angle . . . . ib. QUADBUMAXA. 138. Quadrumana . . . . ,!9 >59- Ourang-Outang ifj 140. Quadrumana climbers . . m 141. Not naturally erect ib. >42- Prehensile tail . . . . in QUADRUPEDS. 143. Quadrupeds .... 124 144. Analogy to the human form . 127 145. Legs and feet .... 128 146. Action of the muscles . . ib. 147. Effect of standing . . . 129 148. Action of the hind legs . . . 130 149. Bounding animals : the kan- garoo ib. 1 5 1. Fossil quadrupeds: Cervus megaceros . . . . >3* 152. Megatherium Cuvieri. . . >33 >53- Mylodou robustus . >34 BIRDS. >54- Animals which fly . . >35 >55- Their locomotive apparatus . 136 156. The bat >37 1 57- Birds ib. >58- Framework of birds ib. >59- Their neck and head . . . >4C 160. Dorsal vertebras ib. 1 61. Birds which do not fly . . >41 162. Locomotive functions of the tail >4> 163. The thorax ib. 164. The breast bone >42 i6j. The clavicles . . . . >43 166. The wings .... >44 167. Their action . . . . >45 1 68. Centre of gravity in birds >6. 169. Magnitude of wings in birds of high flight 146 170. The condor >47 • 7>- Classification : birds of prey. passerine, climbers, gallina- ceous, waders, web-tooted . >4S 172. The legs and feet . . . >5> • 73- Standing >53 • 74- The claws ib. 175- Perching >54 17b. Standing on one leg . . . ib. >77- Climbers >55 1 78. Birds that do not fly . . . 156 1 79. Waders . >57 180. Web-footed birds . . ib. 181. Prehcusile organs of birds •59 182. Goshawk i6e TABLE OF CONTENTS. ?:i Par. Page 183. The kite . . . 160 184. Sea birds . . ib. 1S5. Insectivorous birds . . . 161 186. Granivorous birds . . ib. 187. Pelican . . . 162 188. Hornbill . . 163 189. Fossil birds . . ib. REPTILES. 190. Their form and structure 164 191. Serpents . ( 166 192. Poisonous serpents . . 167 193. Remedies . ib. 194. The skeleton of reptiles . . 169 195. Their skull . , . ib. 196. Trunk . , ib. 197- Skeleton of the tortoise 170 198. Lizards . . . 171 199. The dragon . . . . . 172 200. Extinct reptiles . . *73 201. Fossil saurians . ib. 202. Megalosaurus . . 174 203. Fossil reptiles . . 175 204. The ichthyosaurus . ib. 205. The plesiosaurus . . , 176 206. The cetiosauras . . ib. 207. The pterodactyle • • 177 AMPHIBIA. 208. Habits and structure . 178 209. The seal . • ib. FISHES. 210. Structure adapted for swim* ming . 179 211. Fins .... 180 212. Gills .... . ib. 213. Scales .... . . ib. 214. Skeleton . . . ib. 215. Organs of natation t , 181 216. Air-bladder ib. 217. Flying fish . 182 218. Sucking-fish . . , ib. 219. Mode of propagation . . . 183 220. Migrations . . , . ib. 221. The herring , , 184 222. Periodical voyages . , , ib. 223. Herring fishery . . . 186 224. Dulness of the senses # , 187 215. Electric fishes . ib. 226. Gymnotus electricus . ib. 227. Manner of capturing them . 188 228. Electric organs . . . ib. 229. The torpedo ib. 250. The silurus electricus . . 189 251. Species of electric fishes . . 190 232. Fossil fishes . . . . .191 INVERTEBRATE ANIMALS. 233. Distinction from vertebrate animals , , . , . 193 Par. Page 234. Groups 193 235. Animals of annulose structure ib. 236. Number of feet various . 195 ib. *17- Articulated animals . . . 238. INSECTS. Tegumentary skeleton . . !95 239. Antennaj . . 196 240. Legs and feet . . . . 197 241. Wings ib. 243. Twelve orders of insects . . 198 244. Abdomen 199 245. Metamorphoses . . . . 200 246. Machaou butterfly . 201 247. The silkworm . . . . 202 248. Its importation ib. 249. Cultivation ib. 250. Food ib. 251. Pupa state 203 232. Colour of the silk . ib. 253. Its complete transformation . ib. 254- The day-fly .... 204 256. Its transformation . . . 206 257. Its instincts and end 207 258. The beetle ib. 259. 260. Surprising structure of the larva Incomplete metamorphoses . 208 210 261. Fossil insects . . . . ib. 262. Myriapods 211 263. Groups of this class ib. 264. ARACHNIDA. Structure 212 265. Groups of the class . . . ib. 266. Spiders Spider’s web . . . . ib. 267. 213 268. Classification . ib. 269. Itch insect ib. 270. Fossil arachnida 214 271. CRUSTACEA. Structure 214 272. Food 218 273- Reproduction . . . . ib. 274. Fossil Crustacea 219 275- WORMS. Structure 220 276. Leech 221 277. Fossil annelids. 222 278. ROTIFERA. Wheel animalcule . 222 xii TABLE OF CONTENTS. Par. MOLLUSCA. Pa see 279. General characters . . 223 280. Classification . 224 281. Cephalopods . . ib. 282. Fossil cephalopods . 225 283. Gasteropods . 228 284. Fossil gasteropods . ib. 2S5. Pteropods . 229 286. 287. Aeephala .... Molluscoids . 250 • 288. Tunicata . ib. 289. The bryozoares . ib. 290. Fossil aeephala . . 232 291. The spondylus. . ib. 292. The pentamerus . . ib. 293. The reticulipora . 234 ZOOPHYTES. 294. Structure and classification : echinodermata . . . 234 295. The acalepha, or sea-nettles . 236 296. Medusa ib. 297. Coral animals, or polyps . ib. 298. Coral reefs and islands . . 237 299. Infusoria . . . . . 238 300. Fossil zoophytes . . . . 241 301. The pentacrinus fasciculosus . ib. 302. The eyathina . . . . ib. 303. Theauabacia . . . . ib. CHAPTER V. THE NERVOUS SYSTEM. 304. Nervoussystemconnectsmind and matter . . . . 242 305. Cerebro-spinal and ganglionic system ib. 306-7. General definition of each system 243 THE CEREBRO-SPINAL SYSTEM. 308. Cerebro-spinal axis . . . 243 309. Encephalon and spinal mar- row ib. 310. Membranes of cerebro-spinal axis ib. 31 1. Dura mater .... ib. 312. Arachnoid 244 313. Pia mater ib. 314. Theoretical illustration of ce- rebro-spinal system . . ib. 315. Encephalon .... 246 316. Proportion of parts of ence- plialou 247 317. Average weight of brain . . ib. 318. Comparative weight in dif- ferent animals . . . 248 319. Comparative weight in dif- ferent races . . . . ib. 320. Ditto in different sexes . . 249 Par. Page 321. Weight of brain in remark- able individuals . . . 149 322. Cerebrum or brain . . 250 323. General description of cere- brum ib. 324. Corpus callosum . . . 2ji 325. Septum lucidum . . . . 252 326. Ventricles .... 253 327. Velum interpositum . . . 25; 328. Base of the brain . . . ib. 329. Lobes ib. 330. Cerebrum composed of white matter coated by grey . . ib. 331. Cerebellum .... 256 332. Parts at base of brain . . ib. 333. Corpora quadrigemina . . 257 334. The nerves in general . . 238 333. Nomenclature of nerves . . ib. 336. Cranial and spinal nerves . . 259 337. Cranial nerves . . . . ib. 338. Spinal cord 260 339. Fissures of cord . . . ib. 340. Columns of spinal cord . . 261 341. Spinal nerves . . . . 263 342. Transverse section of cord . ib. 343. Roots of spinal nerves . . ib. 344-5. Vertical section of spine . . 264 346. Classification of spinal nerves 266 347-8. Their structure . . . 266 349. Substance of nervous fila- ments 267 350. Facial nerves .... 268 351. Cervical nerves . . . . 271 352. Brachial nerves . . . 272 353. Crural nerves . . . . 274 THE GANGLIONIC SYSTEM. 334-5. The ganglionic system, or sympathetic nerve . . 27s 357. General view of ganglionic system 277 358. Its origin .... 281 NERVOUS SYSTEMS OF INFERIOR ANIMALS. 359. Importance of the subject . 281 360. General plan of structure . . 2S2 361. Differences in development . i&. 362. Quadrumana . . . . ib. 363. Carnivora .... 284 364. Marsupialia . . . . ib. 363. Rodents 286 366. Birds ih. 367. Reptiles and amphibia . . 287 365. Fishes 288 369-70. lnvertebrata .... 289 371. Radiata 290 372. Mollusca 291 373. Insects 292 FUNCTIONS OF THE NERVOUS SYSTEM. 374. General considerations . . 294 373. Sensibility .... 29s TABLE OF CONTENTS. sin Tar. . . . F»*f 576. Perceptibility . . . . 296 377- Volition ib. 37S. Excitability . . . . ib. 379. Nerves of sensation and of motion ib. 380. Senses ib. 381-2. Tactile sense . . . . 297 383-4. Experiments by which the functions of the nerves are ascertained . . . . ib. 385-7. Effects of ligatures . . .298 388. Flourens’ experiments . . 299 389. Effects of irritation of divided nerve ib. 390. Examples of transferred sen- sation 301 391. Effects of galvanism . . . ib. 392. General conclusion . . . 302 393. Irritation of spinal nerves . ib. 394. Properties of spinal cord . 303 395. Experiments of Bell, Magen- die, Longet, Flourens, and Miiller ib. 396. General conclusions deduced 304 397. Implantation of roots of nerves ib. 398. General results of Flourens’ researches . . . . 305 399. Pathological phenomena . ib. 400. Connection between the spinal cord and the brain . 306 401. Unsettled points . . . 307 402. Keflex nervous action . . ib. 403. Limits of sensibility . . . 308 404. Experiments of Flourens, Haller, Lorry, and Zinn . ib. 405-6. Limit of excitability . . . 309 407. Action of electricity on spinal cord ib. 408. Function of the brain . .511 409-10. Brain, the seat of will and sensation . . . .312 411-14. The brain the seat of the mind 313 41 s. Organic division of labour . 315 416-19. Flourens’ experiments . . ib. 420. Functions of the quadrigemi- nal tubercles . . . 317 421-2. Experiment of Flourens . 318 423-4. Function of the cerebellum . ib. 425. Experiments of Magendie . 319 426. Cross effects . . . . 320 427-30. Functions of medulla ob- longata . . . . ib. 431. The vital point . . . . 321 432. Functions of the ganglionic system . . 322 433- General summary of the ner- vous functions . . . 323 CHAPTER VI. CIRCULATION. 434. Waste of the body . . .326 435. Growth a,. 436. The blood .... ib. Par. Page 437. Its composition . . . . 326 438. Transfusion . . . .327 439. The vital properties of the constituents of the blood . ib. 440. The organs increase according to the blood they receive . ib. 441. Blood is not an homogeneous fluid 328 442. Blood-corpuscles . . . ib. 443. The experiments of Messrs. Donud and Foucault . .330 444. Magnitude of blood-corpuscles ib. 445. White corpuscles . . .331 446. Circulation . . . .332 447. Illustration of the circulating mechanism . . . . 333 448. Great and lesser circulation . 355 449. Red and black blood . . 336 450. Auricles and ventricles. . . ib. 451. The aorta .... ib. 452. Arteries and veins . . .337 453. Lymphatics . . . . ib. 454. Internal structure of the heart 338 455. Course of the blood . . . ib. 456. Valves of the heart . . 339 457. Position of the heart and lungs ib. 458. Bronchial tubes . . . 340 459. General view of the arterial system 341 460. Illustrative diagrams of the valves ib. 461. Position of the heart . . 344 462. Its dimensions . . . 345 463. Structure and distribution of the blood-vessels . . . 349 464. Capillaries . . . .351 465. The pulmonary or lesser cir- culation ib. 466. Pulsations of the heart . . ib. 467. The embouchures of the pul- monary veins . . . 352 468. The contractile force of the cardiac muscles . . . 353 469. The beating of the heart . . ib. 470. Torsion of the heart . . 3 54 471. The force with which the blood is propelled through the arteries . . . . ib. 472. The structure of the arteries ib. 473. The pulse . . . .356 474. The capillaries . . . . 357 475. The veins . . . . ib. 476. The valves of the veins . .359 477. Cicatrisation of wounds . 360 478. The circulation of the blood rendered visible in the microscope .... 361 479. Daguurreotypcs of tho circu- lation ib. 480. General ramifications of tho blood-vessels . . . . 363 481. The skin ib. 482. Blood-vessolsofthe mesentery 365 483. Blood-vessels of the head and lace ■ it). 484. Blood-vessels of the arm and hand 367 XIV TABLE OF CONTENTS. Par. Page 485. Blood-vessels of the leg and foot 368 486. The circulation in the foetus . ib. 487. The circulation in mammifers ib. 488. The circulation in birds . . 369 489. The circulation of reptiles . 371 490. The circulation in the lizard 372 491. The circulatory apparatus of the tortoise . . . . 373 492. The crocodile .... 374 493. Circulation in fishes . . . ib. 494. The circulation of insects . 377 495. The circulation of arachnids ib. 496. The circulatory apparatus of Crustacea . . . . ib. 497. The circulation in mollusca . 378 498. The circulation of zoophytes 379 CHAPTER VII. THE LYMPHATICS. 499. Absorption . . . . 380 500. Illustrated by experiments . ib. 501. Lymphatics . . . . ib. 502. Thoracic duct . . . . 381 503. Right lymphatic vein . . ib. 504. Lymphatic glands . . . ib. 505. The thoracic duct . . -383 506. Analogy of lymphatics to a system of drainage . . ib. 507. Contractile action of the lym- phatics . . . . ib. 508-9. The internal structure of the lymphatics . . . . 384 510. Chyliforous vessels . . ib. 5 1 1. Absorption by the lymphatics ib. 512. The movement of the lymph and chyle . . . . ib. 513. Progression of lymph favoured by muscular motion . . 385 514. Also by mechanical pheno- mena of respiration . . ib. 515. No communication between the lymphatics and veins in the glands . . . . ib. 516. Sources of lymph . . . 386 517. Beautiful structure of the lymphatics . . .« .387 518. The lymphatics of vertebrate animals 389 519. The lymphatics of birds . ib. 520. The lymphatics of reptiles and fishes . . . . ib. 522. Lymphatics of the inverte- brate classes . . . ib. 523. Lymphatics of insects . . 390 524. Lymphatics of radiata . . ib. CHAPTER VIII. RESPIRATION. 525. Respiration . . . . 391 I 526. Seat of the process. . . 391 527. Changes produced by it . . ib. jar. Page 528. Inspiration and expiration . 391 529. Two classes of phenomena in- volved ib. 530. Mechanism of respiration . 392 531. Motion of ribs and sternum . ib. 532. Base of the thoracic cavity, the diaphragm . . . 393 533. Respiratory motion of dia- phragm . . . . ib. 534. Respiration illustrated by a bellows . .... 394 535. Intercostal muscles . . ib. 536. The thorax with its appendages ib. 5 3 7-8. Action of the diaphragm w gentle respiration . . 396 539. Pectoralrespiration in females t b. 540. Action of the intercostal muscles secondary . . ib. 541. Respiratory action of the in- tercostals . . . . ib. 542. Respiratory muscles . . 308 543. Secondary uses of respiration ib. 544- Sighing 399 545. Yawning tb. 548. Laughter ib. 547. Sobbing ib. 548. Lungs and air-passages . . ib. 549. The form of the lungs . . ib. 550. The trachea . . . . 16. 551. The bronchi . . . . ib. 552. Air-cells and intercellular pas- sages 400 553. Lungs not entirely inflated and evacuated of air . . ib. 554. Dimensions of air-cells and intercellular passages . . ib. 555. Sanguiferous capillaries of lungs ib. 556. Physiological effects of respi- ration 401 557. Constituents of atmosphere . ib. 558. The analysis of air expired . ib. 559. Carbonic acid expired . . ib. 560. Aqueous vapour expired . . 402 561. Summary of result of respi- ration .... ib. 562-3. Various theories of respira- tion ib. 564. Theory of Lavoisier . . i'6. 565. Theory of Davy . . . . 403 566. Other hypotheses . . . ib. 567. Hypothesis of Magendie . 404 568. Lagrange and Hassenfratz . i&. 569. Another theory . . . . 405 570. Experiment of W. Edwards . ib. 571. Experiments of Magnus . ib. 572. Objections to these answered 406 573. Source of animal heat . . 407 574-5. Average consumption of oxy- gen per day .... ib. 576. Vegetables consume the car- bon evolved by animals . . ib. 577. Plants injure the atmosphere at night 408 578. Relation between respiration and bodily activity . . 409 579. Respiration of persons of sedentary habits . . . ib. TABLE OF CONTENTS. xv Par. 5S0. Page Gradual change of matter composing the body . 409 581. Interval at which the change is complete . . . . 410 581. Food supplies carbon . . ib. 583. Effect of insufficient food . . ib. 584. Surplus carbon in the food produces fat . . . • 4* 1 585. Sedentary habits produce cor- pulency .... ib. 586. Corpulency checked by exer- cise and labour . . - ib. 587. Use of fat . . • • 4IZ 588. Corpulency developed chiefly in the abdomen . . . ib. 589. Food supplies the fuel . ib. 590. Warm and cold-blooded ani- mals 4r3 591. Hibernating animals . . ib. 592. Respiration of the lower ani- mals 414 593. Respiration of mammifers . ib. 594. Respiration of birds . . 41 5 595. The air sacs . . . . ib. 596. Bronchial tubes communicate with air sacs . . . . ib. Sgr;. Communication not present in all birds .... 416 598. The movements of inspiration ib. 599. Interchange of gases occurs chiefly in lungs . . . 417 600. Distribution of air sacs in re- 603. 604. 60s. 606. 607. 608. 609. 610. spheric in some fishes. . . 42.2 Cause of asphyxia in fishes . ib. Water-cells ib. 61 1. Respiration of mollusca 423 612. Ditto of cephalopoda . ib. 613. Ditto of gasteropoda ib. 614. Ditto of molluscous ace- phala ib. 615. Ditto of insects . . ib. 616. Anatomy of the Nepa . 424 617. Respiration of arachnida 425 618. Ditto of annelida . . ib. 619. Ditto of Crustacea ib. 620. Ditto of zoophytes . . ib. CHAPTER IX. DI0ESTI02T. 621. Waste of the body . 426 622. Repair by nutrition . . . ib. 623. Digestion . . . . ib. 624. Absorption . . . . ib. 625. Alimentary canal . ib Par. Page 626. The phenomena of digestion 427 627. Appetite and hunger . . ib. 628. Circumstances influencing them . . ib. 629. Activity in proportion to cir- culation 428 630. Influence of temperature of inhabited medium . . ib. Effects of want of food . . . ib. Thirst ib. Aliment — its constituents . 429 Mineral substances alone can- not support life . . 430 Animals denominated her- bivorous, carnivorous, and omnivorous .... Nitrogenised and non-nitro- genised aliments . . . Otherwise called plastic and respiratory — examples of each 431 Necessity for nitrogenised constituents . . . . ib. Necessity of the due mixture of these sorts of food . . 432 Difference between animal and vegetable food . Indications of the omnivorous character of the human or- ganism — experiments of Magendie 631. 632. 633. 634. 635. 636. 637. 638. 639. 640. 641. ib. ib. 642. ib. ib. lation to power of flight ib. ised aliment alone cannot View of the lungs of birds ib. support life . . . . 433 Respiration of reptiles . ib. 643. Variety in aliment useful ib. Amphibious reptiles . . . 418 644. Culinary preparations . . ib. Lungs of reptiles . ib. 645. Prehension of food . 433 Effects of heat and cold on 646. Division of food . . . . 434 reptiles . 420 647. Prehension of liquids . ib. Aquatic respiration . . . ib. 648. Mastication . . . . 435 Respiration of fishes ib. 649. Cooking renders mastication Respiration partly atmo- more easy .... ib. 650. 651. 652. 653- 654. 655. 656. 657-8 659. 660. 661. 662. 663. 664. 665. 666. Alimentary canal : theoretical diagram of it, pharynx and oesophagus, stomach and intestines . . . . 436 Position and arrangement of the parts of the alimentary canal 440 Its coats 441 The stomach . . . . ib. Its mechanical action . . 442 Character of stomachic move- ments ib. Eructation . . . 443 Mechanical action of the in- testines . . . . ib. Chemical phonomcna of di- gestion 445 Insalivation .... 446 Mastication . . . . ib. Secretion of saliva . . . 447 Admirable uses of the salivary glands ib. Their number and position . 448 Quantity of saliva sccrotod during meals . . . ib. Saliva 449 XVI TABLE OF CONTENTS. ■ »r. rage 667. Ptyaline 449 668. Starch 450 669. The salivary glands . . ib. 670. Fatty parts of food not affected by saliva : remark- able case at Paris . ..451 671. Stomachal digestion . . 452 672. Gastric juice . . . . ib. 673. Glands which secrete it . 453 674. Its constituents . . . . 454 675. Pepsihe . . . ' . ib. 676-7. ArtiScial digestion . . . 455 678. Non-albuminous matter not dissolved by gastric juice . 456 679. Natural stomachal digestion . ib. 680. Comparison of natural and artificial digestion . . 457 681. Digestibility of food . . . 458 682. Vegetable less digestible than animal food . . . . ib. 683. Aliments incapable of sto- machal digestion . . . 459 684. Time of digestion . . . ib. 685. Experimentsof Dr. Beaumont ib. 686. Digestibility of feculent ali- ments : duration of sto- machal digestion . . . 460 687. Intestinal digestion . . ib. 688. The pancreas . . . . 461 689. The pancreatic j uice . . 462 690. Digestive effects . . . ib. 691. Conversion of feculent matter into grape-sugar . . . 463 692. The liver ib. 693-4 The bile ib. 695. Its analysis . . - . ib. 696. Digestive effects . . . 464 697. . Liquid of the gall-bladder . 464 698. Experiment by a biliary fistula . .... 465 699. Cliylification . . . . ib. 700. Structure of the inner surface of the small intestine . . ib. 701. Intestinal juice: its analysis. 466 702. Collective effects of the di- gestive juices . . . ib. 703. Intestinal absorption . . . 467 704 Ccecal phenomena . . . 468 705. Defecated matter . . . ib. 706. Intestinal gases . . . ib. 707. The food nourishes the blood, and the blood nourishes the body ..... 469 708. Perfection of the machinery of digestion . . . • 47° 709. Constituents of the body . ib. 710. Mammifers 47' 71 1. Mastication . . . . ib. 712. Masticating apparatus varies with the nourishment of the animal : its form in carni- vora 47 1 713. In insectivora . . . • ib. 714. Ill fruzivorn . . . . ib. ■jib. Uses of incisors, canines, and molars . . . . . ib. •jib. Nutrition of the lower ani- mals : proheusion of food . 473 Par. Pa ft 717. Means of drawing in liquid nourishment . ... 474 718. The suckers of insects . . 47 b 719. Digestion of animals gene- rally . ... . . 476 720. Digestive apparatus of mam- mifers . . . . . ib. 721. Ruminants . . . . ib. 722. The paunch . . . . ib. 723. The reticulum . . . . 477 724. The manyplies or omasum . . ib. jib. The rennet or obomasum . ib. 726. The successive transfer of the food through the stomachs . ib. 727. Changes in the alimentary canal of amphibia . 479 728. Connection between the or- ganisation of the animal and the nature of its food : ob- servations of Cuvier . . ib. 729-30. Digestive apparatus of birds 480 731. Digestive functions of rep- tiles 483 732. Fishes — their organs of nutri- tion and food . . . 484 733. Invertebrates — their digestive apparatus 48 b 734 Nutriment of insects : their digestive apparatus, organs of mastication, alimentary canal ib. 735. Digestive apparatus of mol- luscs 48S 736. Digestive apparatus of ra- diata ib. CHAPTER X. ASSIMILATION, SECRETION, THE SKIN, ANIMAL HEAT. 737. Assimilation .... 489 73 S. Return of the lymph . . . ib. 739. Unexplained phenomena . ib. 740. Growth 490 741. Reproduction of mutilated parts ib. 742. Restoration of bones . . 491 743. Restoration of crystalline humour ib. 744 Restoration of cartilages . il 745. Restoration of the brain and nerves 492 746. Restoration of the blood-ves- sels ib. 747. Regeneration in inferior ani- mals ib. 748. Plastic and respiratory con- stituents of food . . . ib. 749. Liebig's table . . . -493 750. Salt ib. 751. Water 49J 752. Absorbing apparatus of di- gestive canal . . . . ib. 753. Absorption and exhalation equal ib. 754 Exhalation ib. TABLE OF CONTENTS. xvu Par. PaSf 755. Cutaneous respiration . . 496 756. Disturbance of exhalation . ib. 757. Cutaneous transpiration . ib. 758. Us importance . . . . 497 759. Waterproof dresses dangerous ib. 760. Pollution of atmosphere . . 498 761. Ventilation . . . ib. 762. Secretions ib. 76J. Secreting organs . . . 499 764. Structure of secreting sur- faces ib. 765. Glands 501 766. Their peculiar properties . . ib. 767. Influence of the nervous sys- tem 501 768. Theory of secretions obscure . ib. 769. The tegumentary envelope . 503 770. The skin . . . ib. 771. Its magnitude . . . . 504 77 2. Its thickness . . . . ib. 773. Artifice of sculptors . . . ib. 774. Structure of the skin . . 505 775. Its thickness . . . . ib. 776. Epidermis — colour — cuta- neous pigment . . . ib. 777. Mucous body . . . . ib. 778. Desquamation — cutaneous maladies — corns . . . 506 779. Uses of the epidermis . . ib. 780. Papillae ib. 781. Papillary nerves . . . 507 782. Sebaceous glands . . . 508 783. Sudorific glands . . . ib. 784. Skin contractile — effect of cold — casting of the skin . ib. 785. Epidermic appendages — quills and bristles .... 510 786. Fur and wool — hair . . .511 787. Plumage 512 788. Animal heat . . . . 513 789. Quantity developed . . ib. CHAPTER XI. THE SENSES. 790. Definition of senses— organs of sense 515 791. Their number . . . . ib. 79*. Position of the organs . . ib. 791- Their structure . . 516 794- Protective accessories . . ib. 795- Local arrangement . . . ib. 796. Touch ib. 797- Special senses . . . .517 798. Manifestation of design . . ib. 799. Organs of taste and smell . ib. 8co. Common principle — mem- brane of the derma — its papill® — pituitary mem- brane. tympanum, and re- „ t*n.a 518 801. Special accessories . . . 519 I 802. Provisions for efficacy . . 520 - CHAPTER XII. TOUCH. Par. Page 803. Tactile sensibility . . . 521 804. Its utility ... . ib. 80;. Its variation . . . . ib. 806. Tactile nerves . . . . ib. 807. Muscular sense . . . 522 808. Conditions of tactile sensi- bility ib. 809. Papillae not supplied with nerves ..... 523 810. Use of epidermis . . . ib. 81 1. Its thickness increased by pressure and friction . . 524 812. Relative tactile sensibility of different parts — experi- ments of Weber . . . ib. 813 Local variation of the tactile sensibility . . . . 52s 814. Appreciation of heat by the touch 526 815. Explauationofthephenomena 528 816. Tactile sense of inferior ani- nials 53* 817. Fur and hair .... ib. 818. The organs of the tactile sense 53* 819. Admirable provision in the animal economy . 533 820. Instruments of prehension . ib. 821. Use of moustaches . 535 822. Birds ib. 823. Reptiles 536 824. Fishes ib. 825. Annulata and Crustacea . ib. 826. Mollusca and radiata . . . ib. CHAPTER XIII. THE SMELL. 827. Structure of the olfactory organ 517 828. Nasal fossae . . . . ib. 829. Pituitary membrane ib. 830. General description of the organ 538 831. Nerves of olfactory and tactile sensibility .... ib. 832. Conditions of sensibility . . 54° 833. Pituitary glands Their secretion . . . . ib. 834. 541 ms. Relation between smell and taste ib. 836. Argument of M. Brillat Sa- vurin ib. 857- Odours produced by effluvia affecting olfactory nerves — subtle character of theso cffluvia 542 838. Bodies may be impregnated with odorouseffluvia — effect of air passing through the nasal fossio . . . . <41 b TABLE OF CONTENTS. xviii Par. Page 839. Impact on pituitary mem- brane 543 840. Smell soon deadened by excess 544 841. The nostrils ib. 841. Different susceptibility of smell ib. 843. Subjective olfactory sensa- tions 545 844. Direction of odorous objects . ib. 845. Olfactory sense of inferior ani- mals.— Mammifers . . . ib. 846. Birds ib. 847. Reptiles 546 848. Fishes . . . ib. 849. Annulate, mollusca, and ra- diata ib. CHAPTER XIV. TASTE. 850. The tongue .... 547 851. Experiments on taste . . ib. 852. Effects of mastication . . 548 853. Limit of gustatory sensibility 549 854. Delicacy of taste varies . . ib. 855. Experiments . . . . ib. 856. Papillary structure of tbe tongue ib. 857. Dorsal surface . . . ■ 550 858. Calyciform papillae . . . ib. 859. Foramen coecum . . . ib. 860. Fungiform and corolliform papillae 551 861. Hemispherical papillae . .552 862. Distribution of papillae . . ib. 863. Microscopic appearance . . ib. 864. Sensibility of the tongue . 553 865. Lingual nerves . . . . ib. 866. Their terminal form . . . 555 867. Vascular apparatus. . 556 868. Seat of gustatory sensibility . ib. 869. Experiment of Pauizza . . ib. 870. Function of the glosso- pharyngeal nerve . . . ib. 871. Sense of taste of inferior ani- mals .... • 557 872. Tongue of mammifers . . ib. 873. Birds .... . ib. 874- Reptiles . . ib. 875. Fishes .... • 553 876. Iuvertebrata . . ib. CHAPTER XV. VISION. 877. Importance of sense of sight 558 878. The eye an interesting object of study .... ib. 879. Visual apparatus in the higher animals ib. Par. Parr 880. Structure of the eye . . 562 881. The cornea . . . . ib 882. Choroid 561 883. Retina ib. 884. Crystalline .... 562 885. Iris ib 886. Pupil ib. 887. Aqueous humour— crystalline lens 563 888. Vitreous humour . . . ib 889. Eyelids 564 890. Eyebrows and other accesso- ries ib. 891. Numerical data of the struc- ture ib. 892. Motor apparatus . . . ib. 893. Limits of the play of the eye- ball 565 894. Ocular image . . . 566 895. Inverted picture . . . 567 896. Anatomical section of eyeball 568 897. Magnified section of iris . . ib. 898. Eye achromatic and aplanatic ib. 899. Other analogies to an optical instrument . . . . 569 900. Conditions of perfect vision . 57c 901. Distinctness of image . . ib. 902. Near and distant objects of vision 571 903. Optical centre . . . . ib. 904. Remedies for defect of sight . 572 905. Adaptation to different dis- tances ib. 906. Experiments on vfcion at dif- ferent distances . . . 573 907. Visual magnitude . . . 574 908. Minuteness of ocular pictures ib. 909. Ocular image of a man . . 575 910. Sufficiency of illumination . ib. 91 1. Foramen centrale and limbus luteus, or yellow spot . . 576 912. Field of vision . . . 577 913. Its limits 579 914. Attention necessary to per- ception ib. 915. Binocular vision . . . 580 916. Perception of colours . . 5S1 917. Colour bliudness — cases re- corded by Sir David Brew- ster 581 918. Case of Dr. Dalton . . 582 919. Visual organs of inferior ani- mals 5S3 920. Vertebrata ib. 921. The lachrymal apparatus ab- sent in certain classes . . ib. 922. The choroid in animals . . 5S4 923. The iris ib. 924. The pupil, its varying forms ib. 925. The pecten, or marsupium . ib. 926. Microscopic structure of the retina . . ib. 927. Eves of animals which prey by night . . ". 58 s 928. Relation of the humours of the eye to the surrounding medium ib 929. Direction of the optic axis . 586 TABLE OF CONTENTS. xix Par. Pase 930. Eyes of birds . . . . 586 931. Their adaptation to the vary- ing wants of the animal . ib. 932. Eyes of reptiles . . . 587 933. Eyes of fishes . . . . ib. 934. Auuuiata — compound eyes, their structure . . . 588 935. Illustration of compound eyes ib. 936. Number of eyes of different insects 589 937. Eyes of arachnida . . . ib. 938. Molluscous cephalopoda . . 590 939. Gasteropoda .... ib. CHAPTER XVI. HEARING. 940. The theory of the ear not so satisfactory as that of the eye 591 941. Three compartments of the ear ib. 942. The external ear . . . ib. 943. The external meatus . . 592 944. Membrane of the tympanum ib. 945. The middle ear . . . . 593 946. The Eustachian tube . . ib. 947. Fenestra oralis and fenestra rotunda ib. 948. The auricular bones — their use 594 949. Internal ear . . . . 595 950. The labyrinth . ib. 951. The vestibular and cochlear nerves — membranous canals ib. 952. Forms of the membrane of the tympanum — auricular bones 597 953. The auditory nerve . . ib. 954. The osseous or bony labyrinth — and the membranous labyrinth . . . . 598 95J. Semicircular canals . . ib. 956. The cochlea . . . . 599 957. The vestibule .... 600 958. The membranous vestibule — the utricle and sacculus . 600 959. Endolymph and otoliths . ib. 960. Perilymph ib. 961. The acoustic nerve . 601 962. Its distribution in the cochlea ib. 963. The failure of theory . . . ib. 9^4- Acoustic properties of the ex- ternal ear .... 602 965- Artificial aids to bearing . . 603 966 Ear-trumpet . . . . ib. 7 Teeth . Sappey. 44 22 Vertebral column . Ditto. 54 13 Sacrum Ditto. 55 25-7 Intervertebral cartilages . Quain. 56-7 18-9 Vertebral ligaments . Sappey. 58- 9 59- 60 30-2 Atlas and axis Ditto. 33 Base of skull . Ditto. 61 34 Thorax Quain. 62 35 Scapula Ditto. 63 36 Humerus . Ditto. 64 38 Fore-arm Ditto. 66 39 Bones of wrist Sappey. 67 40-2 Ligaments of arm and hand Ditto. 68-0 43-5* Bones of leg and foot Ditto. 7**5 53 Muscles of head and face Ditto. 87 54-6 Ditto of trunk Ditto. 90-4 57-68 Ditto of arm and hand Ditto. 95-106 69-75 Ditto of leg and foot Ditto. 107-12 76-8I Facial angle . Edwards. 115-16 82-5 Different human races Rbgne Animal, Cuvier. 117-is 86 Skeleton of ourang-outang . Ditto. 120 87-88 Ourang-outang Ditto. 122-5 89 White-throated monkey Ditto. 124 9° Kami .... D'tto. 125 91 Mandrill Ditto. 125 91 Skeleton of cam el . Ditto. 126 93-4 Hoofs .... Edwards. 12S-9 95-6 Kangaroo .... Rbgne Animal, Cuvier, >3> 97 Jerboa, or jumping-mouse . Ditto. >31 98 Cer vus megaceros (fossil) . D’Orbigny. >31 90 Megatherium Cuvieri (fossil) . Ditto. >33 1 00 Mylodon robustus (fossil) . Owen. >34 101-2 Bat .... Rhgne Animal, Cuvier. 136-7 103-4 Skeleton of vulture . Ditto. 13S 105 Crane .... . Ditto. >39 106 Flamingo .... . ] Ditto. >39 107 Skeleton of ostrich . Ditto. >41 108 Cassowary .... Ditto. >42 109 Clavicles and sternum of bird Edwards. >43 1 10 Wing feathers Ditto. >45 111 Yellow vulture Rfegne Animal, Cuvier. 146 1 12 Lamb vulture Ditto. 146 in Royal eagle Ditto. >47 114 Frigate bird . Ditto. >47 115-26 Beaks and claws • Ditto. 14S-SI LIST OF PRINCIPAL ILLUSTRATIONS. XXlll Figure SUBJECT. Authority. 127 Ibis ...... Rfegne A7iimal, Cuvier. 128 Penguin . . . . . Ditto. 129 Stork ..... Ditto. 130 Parrot . . . . . . Ditto. '3' Woodpecker .... Ditto. 131 Snowy partridge . . . . Ditto. '33 African ostrich .... Ditto. '34 Long-legged plover or stilt-bird Ditto. '35 King duck ..... Ditto. 136 Eider duck . . . . . Ditto. '37 Head of falcon .... Ditto. 138 Goshawk . . . . . Ditto. '39 Kite of Carolina . Ditto. 140 Martin pecker . . . . . Ditto. 141 Bee eater ..... Ditto. 742 Goat-sucker . . . . . Ditto. '43 Sparrow ..... Ditto. '44 Pelican . . . . . Ditto. '45 Horn-bill ..... Ditto. 146 Fossil foot-prints and marks of rain-drops D’Orbigny. '47 Fossil bird, found in gypsum of Mont- martre ..... Ditto. 748 Green lizard . Rhgne Animal, Cuvie7\ '49 Gi’eek tortoise Ditto. 150 Chaleis . . . . . Ditto. 'S' Raja aspic . Ditto. 152 Mouth of rattles77ake Edwai-ds. '53 Rattlesnake Ditto. '54 Skeletonof head of ditto Ditto. '55 Skeleton of tortoise Ri'gue Animal, Cuvier. 156 Gecko . . . . . Ditto. '57 Sea tortoise Ditto. 158 Dragon Ditto- •59 Ichthyosaurus (fossil) Edwa7-ds. 760 Plesiosaurus (fossil) . . . Ditto. l6l Cetiosaurus (fossil) D’Oi-bigny. 162 Pterodactyle (fossil) . . . . Ditto. 163 Seal . Rbgne Animal, Cuvier. 764 8keleton of ditto Ditto. 165 Skeleton of fish Ditto. 766 Flying fish Ditto. 767-8 Remora, or sucking fish . Edwards. I 70-1 Torpedo . . ... Ditto. 172 Silurus electricus Ditto. •73 Lebias cephalotes (fossil) D’Oi-bigny. •74 Platae altissimus (fossil) . Ditto. •75 lulus . Edwa7-ds. I76 Scolopendra . Ditto. •77 Spider . Ditto. • 78 Skeleton of grasshopper . Ditto. •79 Carabus or land beetle Ditto. 780 Marine necrophorus Ditto. 787 Ichneumon Ilhgne Animal, Cuviei". 182 Plumed moth Ditto. 183 Ant lion . . Ditto. 783 a and b Abdomen and sting of bee 184 Caterpillar of Machaon butterfly Edwards. '8; Chrysalis of ditto . Ditto. 186 Machaon butterfly Ditto. • 87 Silkworm on mulberry leaf Ditto. 788 Chrysalis of ditto Ditto. 789 Moth of ditto . Ditto. 790 Ephemera or day-fly Ditto. • 91 Larva of ditto . Goring. 792 Same, magnified . Ditto. •93 Magnified view of dorsal ycssc! Ditto. l*nge. IS2 • 53 •54 155 •55 156 156 •57 158 158 '59 '59 160 161 161 161 161 162 162 163 164 165 165 165 t 66 167 168 169 170 171 172 172 176 176 177 '77 .78 179 181 182 182-3 188-9 I89 191 192 !95 194 J94 196 196 196 197 1 97 x99 200 201 202 202 203 205 203 204 204 205 206 XXIV LIST OF PRINCIPAL ILLUSTRATIONS. Figure. SUBJECT. Authority. Page. •94 Beetle Goring. 208 195 Its larva in the natural size, called the water-devil . Ditto. 2C8 196 The same magnified Ditto. 209 '97 Fossil dragon-fly D’Orbigny. 211 198 Mygale (spider) .... Edwards. 212 199 Itch insect . . . . Mandl *>3 200 Fossil scorpion .... D'Orbigny. 114 201 Crab . . . . . Rtgne Animal, Cuvier. 202 Spiny lobster Ditto. 2l6 203 Squilla . . . . . . Ditto. *'7 204 Cray-fish ..... Edwards. 218 205 Its masticating apparatus Ditto. 218 206 Prawn .... Ditto. 219 207 Ogygia guettardi (fossil) D'Orbigny. 220 208 Nereis Edwards. 220 209 Leech . . . . . . Jones 221 210 Serpulse .... Edwards. 221 21 1 Serpula flagellum (fossil) D’Orbigny. 222 212 Nereites Cambriensis (fossil) Ditto. 222 213 Rotifera .... Mandl. 113 214 Poulpe ..... Edwards. **S 215 Paper nautilus . . . Ditto. 226 216 Nautilus danians (fossil) . D’Orbigny. “7 217 Ceratites nodosus (fossil) Ditto. 227 218 Ammonites humpriesianus (fossil) Ditto. 1*7 219 Murchisonia bigranulosa (fossil) Ditto. 228 220 Cyprea elegans (fossil) Ditto. 228 221 Voluta elongata (fossil). Ditto. 229 222 Pteroeera oceani (fossil) . Ditto. 229 223 Anatomy of oyster . . . . Edwards. 230 224 Shell of pearl oyster Ditto. 230 225 Plumatellae . . . . . Ditto. *31 226 Spondylus spinosus (fossil) D'Orbigny. *3* 227 Pentamerus Knightii (fossil) Ditto. *33 22s Reticulipora obliqua (fossil) Ditto. *33 229 Holothuria . . . . Edwards. *34 23O Sea-urchin ..... Ditto. *35 231 Star-fish . . . . . Ditto. *35 232 Medusa ..... Ditto. *36 *33 Polypes . . . . . Ditto. *37 234 Coral island .... Beudant. 138 Pentacrinus fasciculosus (fossil) D’Orbigny. *59 236 Cyanthina Bowerbankii (fossil) . Ditto. *40 237 Anabacia orbulites (fossil) . Ditto. 240 238 Nervous system of man . Edwards. *45 239 Encephalon . . . . . Sappey. 246 240 Vertical section of ditto . Hirschfeld and Lcveille. *5' 241 Cerebrum viewed from above, showing the corpus callosum Foville. *5* 242 Section of encephalon, showing the floor of the lateral ventricles • . Hirschfeld and Lcveille. *53 243 Velum interpositum, lying between the fornix and the parts beneath . Sappev. *54 244 Inferior surface of the encephalon . Hirschfeld and Lcveille. 156 245 Corpora striata and adjacent parts Sharpey. *57 246 Spinal nerves . . . . . Edwards. *58 247-9 Spinal cord .... Hirschfeld and Leveilld. 26l 250 Constituent cords of spinal marrow Ditto. 26l 251 Transverse section of the spinal column Ditto. 263 252-3 Connection of spinal nerve with cord . Sharpey. 264 254-6 Longitudinal section of spinal cord and nerves . . . . Hirschfeld and LeveillA 265 257 Structure of nerve Bell. *67 258 N erves of face and head Hirschfeld and Lcvcilld. 269 259 Cervical nerves .... Ditto. *7° LIST OF PRINCIPAL ILLUSTRATIONS. XXV Figure. SUBJECT. Authority. Page. 260 Brachial plexus . . . Hirschfeld and Leveill6. 261 Ditto nerves .... Ditto. 261-5 Nerves of arm and hand Ditto. 266-9 Ditto of leg and foot Ditto. 270 General view of the assemblage of the parts of the ganglionic system, showing its connection with the principal cephalic and spinal nerves Ditto. *71 General view of the principal nerves of the neck ..... Ditto. 272-3 Encephalon of the ourang-outang . Treviranus. *74 Ditto of the lion .... Leuret. *75 Ditto of the seal . . . . Ditto. 276 Ditto of the hedgehog Ditto. *77 Ditto of the mole . . . . Ditto. 278 Ditto of the bat .... Ditto. *79 Ditto of the opossum. Owen. 280 Ditto of the beaver Ditto. 281 Ditto of the giraffe . . . . Ditto. 282-4 Ditto of the fowl . Rfegne Animal, Cuvier. 185-7 Ditto of the crocodile Ditto. 288-90 Ditto of the adder Ditto. *91-3 Ditto of the frog Ditto. 294-6 Ditto of the perch .... Ditto. *97 Nervous system of the star-fish Muller, after Tiedemann. 298 Ditto of the black slug Ditto. 2990 Ditto of the sphynx ligustri . M filler. 301 Ditto of the field-beetle . Ditto. 302 Ditto of the earwig . . . . Edwards. 3°3 Ditto of the grasshopper . Ditto. 304 Ditto of the stag-beetle Ditto. 3°5 Ditto of the field-bug Ditto. 309 Globules of the human blood magnified and daguerreotyped Donne and Foucault. 310 Diagram of the circulation of the blood . Gardner. 3" Theoretical section of the heart . Edwards. 31* Heart and lungs . . . Ditto. 3*3 Trachea and branchial tubes Ditto. 314 Arterial system . . . Ditto. 315 Valves of the heart Ditto. 316 Structure of ditto . . . . Sappey. 3'7 Muscles of the auricles Ditto. 3.8 Ditto of the ventricles Ditto. 3'9 Front view of the heart . Ditto. 320 Back view of ditto . . . Ditto. 3*1 Vertical section of heart through right auricle . . . . Ditto. 3** Ditto through right ventricle Ditto. 3*3 Ditto through left ventricle and auricle Ditto. 3*4-5 Valves of veins . . . . Edwards. 326 Circulation of blood in the foot of frog . Ditto. 3*7 Magnified daguerreotype showing the 3*8 circulation in the tongue of a frog Donne and Foucault. Blood-vessels of the mesentery Sappey. 329 Ditto of the head and face Ditto. 330-1 Ditto of the arm and hand . Ditto. 331-3 Ditto of the leg and foot . Ditto. 334 Circulation in mammifers and birds Edwards. 335 Arterial system in birds . Ditto. 336 Circulation in reptiles Ditto. 337 Ditto in the lizard Ditto. 338 Ditto in the tortoise . . . . Ditto. 339 Ditto in the crocodile Ditto. 340-1 Ditto in fishes . Ditto. 34* Ditto in Crustacea .... Ditto. 343 Ditto in a snail . . , Ditto. 271 272 2-73-4 175-6 i77 280 2S3 283 284 284 284 284 284 284 284 286 287-8 288 288 289 291 291 292 293 294 294 294 294 329 334 339 340 341 341 343 343 344 345 346 347 348 349 3 S° 359 361 362 364 365 366 367 369 370 37< 37* 373 374 375 378 379 XXVI LIST OF PRINCIPAL ILLUSTRATIONS. Figure. SUBJECT. 344 345 346 Lymphatic gland . . . . Thoracic duct and great lymphatic trunk 1 Section of lymphatic vessel showing the valves . . . . . , 347 348 349 3 S° 354 356 357 358 $ 361 362 363 364 365 Chylifers of the mesentery . Lymphatics of the trunk and head Lymphatics of the arm and hand . Ditto of the leg and foot . Thorax and its appendages . Marmot ..... Lungs of a bird . . . . Organisation of the adder Hippoeamp, or sea-horse Shark ..... Lamprey .. . . . Anabas ..... Nepa, or water-scorpion Arenicola, or sandworm . Diagram of the alimentary canal in the human organism 366 367 368 369 37° Pharynx and cesophagus Stomach and intestines . Ditto ditto . . . . Ileum and large intestine First appearance of the parotid gland in 37' 373 374 375 376 377 378 379 380 381 382 383-4 385 386 388-9 390 391 392 393 394-5 396 397-8 399 400 401 402 403 4°4 4°5 406 407 409 410 41 1 412 413 414 a sheep . . . - Lobules of gland in a more advanced stage ..... Stomach, liver, and pancreas Inner surface of small intestine magnified Lion’s tooth .... Teeth and jaws of carnivorous animal . Ditto of insectivorous animal Ditto of frugivorous animal Ditto of graminivorous animal Jaws of the boar . Ouistitti it pinceau ( jacchus penicillatus) Elephant’s trunk . . . . Carabus ..... Calamary . . • • Hydra or fresh-water polyp Stomachs of a sheep . . ■ Tongue of a bird . Head and tongue of a woodpecker . Digestive apparatus of domestic fowl . Buccal apparatus of the wild bee Termes fatalis, or white ant . Digestive apparatus of insects Theoretical diagrams of secreting surfaces Parotid gland . . . • Kidney . Papillae of sldn. ■ • • Embouchures of excretory ducts mag- nified . . ■ • - Sudorific gland . . • • Root of a hair . . • • Porcupine . Squirrel . • • • • Pangolin, or scaly ant-eater Indian elephant . • • Head of a tapir . Shrew-mouse . . • • Giraffe , External wall of left nasal fossa covered with pituitary membrane . • • Nerves distributed on the internal wall Authority. Fa**- Sappey. 38! Mascagni. 382 Sappey. 384 Edwards. 38b Ma.'cagni. 387 Ditto. 388 Ditto. 388 Edwards. 395 Rt'gue Animal. Cuvier. 4H Edwards. 4>7 Ditto. 4>9 Ditto. 420 Ditto. 421 Ditto. 421 Ditto. 422 Ditto. 424 Ditto. 425 Lardner. 437 Edwards. 438 Quain. 439 Edwards. 440 SantorinL 444 Mliller. 448 Ditto. 449 Tiedemann. 461 Boehm. 465 Edwards. 47* Ditto. 47- Ditto. 471 Ditto. 472 Ditto. 4-2 Ditto. 473 Ditto. 473 Ditto. 474 Ditto. 474 Ditto. 475 Ditto. 475 Ditto. 4-8-9 Ditto. 4S1 Ditto. 4S1 Ditto. 4S2 Ditto. 4S5 Sraeathman. 4S6 Edwards. 487 MUller. 500 Edwards. 501 Ditto. 501 Breschet. 507 Ditto. s9 Nerves connecting the eye with the brain Edwards. 561 420 Large section of the eye . Quain. 562 421 Structure of the iris . . . . Ditto. 565 4ZZ Motor muscles and lachrymal gland oi the eye ..... Sappey. 56s 411 Motor muscles . . . Ditto. 566 424 Experimental exhibition of picture on retina ..... Ditto. 567 425 Vertical section of left eye Ditto. 568 426 Segment of iris magnified Ditto. 569 4^7 Arteries of iris . . . . Arnold. 570 4*9 Section of the retina of a crow . Treviranus. 585 430 Section of the eye of a cockchafer . Strauss-Durckheim. ;88 45' Segment of the same Muller. 588 451 Cornea of a compound eye of the common fly magnified . . . . — 589 455 Section of the eye of scorpion magnified Muller. 590 454 The ear ..... Quain. 59‘ 455 General view of its internal structure . Valentin. 592 456 Section of the cochlea and canals Ditto. 596 437-8 Bones of the tympanum . Arnold. 597 459 Ditto . . . . . . Edwards. 598 440-1 Labyrinth ..... Sommering. 599 441 Section of the cochlea Quain. 600 445 Distribution of nerve in cochlea . Arnold. 601 444 Perspective view of spiral lamina, with auditory nerve distributed upon it Breschet. 602 445 Cochlea of the owl Quain. 606 446 Theory of the glottis Beclard. 609 447 Profile of the larynx . Edwards. 610 448 Section of ditto Ditto. 6l I 449 Front view of ditto . Ditto. 612 450-1 Thyroid cartilage . Sharpey. 612 451 Interior of larynx Willis. 613 455 Side view of ditto . Ditto. 614 454 Trachea dissected Sharpey. 61 5 455-6 Experimental illustration of properties of larynx . Beclard. 616-1S 457 Tongue and glottis of granivorous bird . Edwards. 621 4S8-9 Larynx of crow Ditto. 6zz 460 Ovary of the pig . Ponchet. 626 Graafian follicle and ovum of a mam- mifer . Beclard. 627 462 Ditto, ditto MUller. 627 463 Ovum of a sow Beclard. 628 464-6 Segmentation of mammifer's egg Ditto. 629 467-8 Development of mammifer's egg Ditto. 632 469-70 Ditto at more advanced stages Ditto. 634-5 47' Germinal membrane of ovum of dog BischofT. 6j8 47i Ditto at a more advanced stage Ditto. 639 475 Primitive embryonic circulation of mam- mifer MUller. 474 Primitive circulation of the dog BischofT. 646 475 Second embryonic circulation of mam- miler Beclard. 476 Structure of the embryonic heart Baer. 477 Ante-natal embryonic circulation Sharpey. 650 478 Bird s ovary Carus. 6j6 479 Bird’s calyx fully developed with blood- vessels and stigma Baudrimont and Martin. 6 <6 480 Same with stigma open . Ditto. 657 XXV111 LIST OF PRINCIPAL ILLUSTRATIONS. Figure. SUBJECT. Authority. Vhte. 481 Theoretical section of lien’s egg at com- mencement of development Baudrimont and Martin. 659 482 Transparent and vascular area Ditto. 660 483 Vascular area, showing primitive vitel- line circulation . . Ditto. 663 484-9 Sections of hen’s egg in several progres- sive stages of development Ditto. 665-70 49°-4 Progressive development of embryo of chick ..... Ditto. 671-2 495-6 Embryonic circulation of chick Ditto. 672 497 Embryo chick on 19tli day Ditto. 678 498-536 Segmentation of frog’s egg . M tiller. 680 507-1 1 Ditto of ascarides .... Kolliker and Bagge. 683 512 Reproduction of polyp Trembley. 68j 5'3 Ditto of nais .... M tiller. 684 514-17 Ditto of vorticella . . . . Ehrenberg. 685 518 Daguerreotype of cow’s milk magnified Donn6 and Foucault. 686 51 9 Ditto of woman’s milk Ditto. 686 520 Scale of human growth . Quetelet. 689 ERRATA. Page 38, paragraph. 65, line 25, omit “towards the temples.” Page 41, paragraph 70, line 14, for “C” read “S.” Page 46, paragraph 75, line 15, for “canine” read “canines.” Page 196, fig. 179, for “Carrabus” read “Carabus.” Page 219, Note ought to be * “ Edwards.” Page 234, ,, ,, ,, Page 409, paragraph 580, line 1, for “comprising” read “composing.” Page 601, paragraph 962, line 15, for “ then are ” read “ is then.” Page 602, fig. 444, for “ Sappey ” read “Breschet.” Page 603, paragraph 966, line 9, for “great” read “greatly.” Page 612, paragraph 987, lines 2 and 3, for “and on the natural larynx dissected ; from the dead subject as well as observations,” &c., read “ and on the natural larynx dissected from the human subject ; as well as observations,” &c. Page 638, line 6, for “ large” read “larger.” ■ ANIMAL PHYSICS. CHAPTER I. GENERAL VIEW OF THE ANIMAL ORGANISATION. 1. The diffusion of knowledge is ever commensurate with the progress of discovery. As the boundaries of science are enlarged the craving of the general mind for the acquisition of it is stimulated, and this appetite is rendered more irresistible by the contemplation of the vast benefits and augmented powers conferred upon the arts of life by every fresh advance in scien- tific researches. Who that beholds the prodigious extension given to metallurgic industry by the discoveries of mineralogists and chemists, can resign himself to absolute ignorance of the materials of which the globe is composed, and the elements of which bodies are constituted ? Who that contemplates the marvellous history of the earth revealed by geologists can rest in contented ignorance of the science which has taught us to interpret the characters written upon its crust, in which its state, myriads of ages before it became the habitation of the human race, is recorded ? But of all the stores of knowledge thus spread before us, there are surely none which should more pique our natural curiosity than those which open to us a view of animated nature. In these we have, so to speak, a selfish interest. So far as relates to our material being, we are ourselves one of the order of animals, among which we hold the highest place. Our organi- sation includes the most exalted specimen of nature’s work exhibited to us here below. In no department of science has so extraordinary a body of knowledge been evolved. In none have results been obtained manifesting in a more striking manner the attributes of the Great Maker of All, than in the provisions made for the maintenance and continuance of the n 2 ANIMAL PHYSICS. species which inhabit the earth, and for the moral and physical supremacy of man over them. In all educational institutions established within the last half-century, provisions have accordingly been marie for the supply and enforcement of this branch of general instruction. The regulations of the University of London, as well as those of similar establishments in other enlightened countries, require an acquaintance with the structure and functions of the animal, and more especially of the human, organisation, as a qualifica- tion for the first degree claimed by a student, a degree which signifies the acquisition of the elements most essential to a liberal education. This does not mean, however, that every youth sent forth into the active walks of life is to be a naturalist, aDatomist, . or physiologist. To sound the depths of these sciences would require time and labour incompatible with the other acquirements which must be attained by such students, and would include a multitude of details useful to those alone who propose to make these sciences their specific study. It is not necessary to a liberal education to be able to pronounce upon the nice distinctions which separate species from species, nor to trace the course of each artery and nerve which traverses the organs of the body. Such, details would encumber the memory of the general student, without leaving upon it any durable or useful traces. The knowledge which is desired is of another kind. The general structure of the human body and its organs, the principal functions by which they are sus- tained and nomished, the most striking relations and analogies between them and the corresponding organs of inferior species, the manner in which the functions are exercised, and the relations between the varying structure of the organs and the habitudes of life of the species to which they belong, constitute knowledge which once well and soundly acquired can never be forgotten, and which, while it suffices for those whose pursuits in life do not necessarily connect them with the prosecution of the natural sciences, serve as the base of the special studies of those who engage in professions, with which these sciences are necessarily connected. Such is the purpose which the present volume pretends to attain. While, however, these are the immediate and only direct subjects of instruction included in its pages, other in- direct, though scarcely less important, advantages to the student may be hoped for. The influence which the study of natural science exercises upon the intellectual faculties merits GENERAL VIEW. 3 serious attention. The course of investigation through which the mind is conducted in such studies habituates it to ascend from effects to causes, yet never advancing a step without submitting the deductions of reason to the severe tests of experiment and observation. While such studies lead, there- fore, to a habit of lofty speculation, they never permit the imagination to wander, inasmuch as the material verification is rigorously placed in juxtaposition with the speculative hypothesis. This, in a word, like every other branch of natural history prosecuted on just principles, exercises the mind better than any other in those methods of reasoning, without which all investigation is laborious and all exposition obscure. Many works have been composed by eminent writers on sub- jects analogous to those treated of in the present volume, with the express and exclusive view of illustrating the divine attributes by the exposition of the never to be too much admired provisions made in the organised world for the well-being of the creatures which inhabit it. Although such has not been the object with which these pages have been composed, the reader will soon perceive that it has not been overlooked, and it may be hoped that such reflections expressed when they arise naturally out of the exposition of the phenomena, will not have the less force, inasmuch as they are not obtruded in the mere spirit of advocacy. The relations between the phenomena developed in the animal organisation and the laws of physical science are insisted on more especially, and the structure, organisation, and functions of the animal body, are viewed as beautiful examples of the play of the same principles which are brought into action in an infinitely less perfect manner in the artificial contrivances of man, more frequently than is generally the case in treatises on natural history. 2. Structure of the Body. — The animal body consists of solids and fluids. The solids, in all except the lowest species, are, some hard and compact ; others soft, flexible, elastic, or tough, in various degrees. The hard and compact parts serve as the frame-work and support for the others. They give form and external character to the body, and precision to its mo- tions. In certain inferior species, these hard parts constitute the external covering, and enclose and protect the softer and more delicate organs. In such cases they are called the shell. b 2 4 ANIMAL PHYSICS. In all tlae higher species, however, without exception, the hard and compact frame-work of the body is within its exterior covering, the softer parts being, some attached to and supported by it, as well outside as inside, and others deposited in suitable cavities adapted for their conservation and the special exercise of their respective functions. The hard and compact parts, which thus form the frame-work of the body, are in such species called the bones. 3. The principal fluids are included in central reservoirs, from which they are propelled by a suitable apparatus to the extremities, and driven back from the extremities to the central reservoirs through a system of flexible tubes, which vary in their cabbre from the large conduits leading from and to the central reservoirs to capillary tubes so minute as to be micro- scopic. The fluids thus kept in circulation distribute to all parts of the system whatever is necessary to restore waste in the adult, and to supply growth in the young. It will be apparent that in such an organism must be found the play of the principles of every branch of physical science. The bones are constructed and moved upon the same principles of mechanics as govern all machinery. The fluids are propelled and distributed by an apparatus illustrating the principles of hydrostatics and hydraulics. The organs of re- spiration illustrate the principles of pneumatics ; the sense of vision is exercised by an optical instrument of admirable per- fection ; and the sense of hearing and the organs of voice are examples of the most perfect acoustic apparatus. 4. Skeleton. — The assemblage of bones, in their natural order and juxtaposition, is called the skeleton, from the Greek word o-/ceXXo> (skello), to dry, the bones thus exhibited and combined being previously stripped of the flesh and dried. 5. Mechanism of the Skeleton. — The motions of which different parts of the body are susceptible have so close an analogy to those of certain parts of machinery, that it might be naturally supposed that the moving parts should be in both cases connected by the same expedients. Thus, for example, the motions of the arm upon the shoulder, and of the thigh upon the hip, have a play and limits altogether similar to that produced by the ball and socket-joint, while the motions of the fore-arm at the elbow, and the leg at the knee, are perfectly similar to that of the hinge or cradle-joint. Never- GENERAL VIEW. 5 theless, although the mechanical connection of these members has something in common with the expedients referred to, they are far from being identical with them. In machinery, the moving parts are generally connected by inflexible pieces ; thus, for instance, the parts connected by a ball and socket- joint are firmly held together by the enclosure of the ball in a hollow spherical cavity somewhat greater than a hemisphere. As the ball cannot pass through an opening having a diameter less than its own, it cannot escape from such a socket. If the socket were only equal to, or still more, if it were less than a hemisphere, the ball would be capable of moving in, but would not be retained by it. In like manner, two parts connected by a hinge are retained in their relative positions by a rod, pin, or wire passing through a hole made in the common centre of the two pieces which more immediately form the joint. If this rod or pin were withdrawn, the parts would still be capable of moving in the manner permitted by the joint, but would not be retained in their proper juxtaposition. 6. If the bones composing the skeleton were held together at their several joints by mechanical expedients similar to those used in machinery, it would follow that, were it possible sud- denly to divest them of all the flesh and softer parts, leaving nothing but the hard and compact substance properly called bone, the skeleton would still maintain all its parts in their proper relative positions, and each would still be susceptible of the same motions ; for by the supposition, the joints being all formed by mechanical connections similar in material to those of the bones themselves, would not be affected by the removal of the softer parts with which they were previously surrounded, and to which they gave support. 7. Such, however, would not be the case. On the contrary, the sudden dissolution and removal of all parts of the body not strictly osseous would be instantly followed by the complete dismemberment of the skeleton, the various bones of which, with a few exceptions which will be noticed hereafter, would fall asunder by their own gravity. 8. It appears, therefore, that if the skeleton afford support to the softer parts of the system, these latter must reciprocally give support to it. Neither could maintain their natural position unsupported by the other. 9. Joints. — The joints, or articulations as they are tech- nically called, unlike those in artificial mechanical combinations, 6 ANIMAL PHYSICS. are formed not by sockets like those of ball and socket- joints ; nor by rods or pins like those of cradle-joints ; nor, in short, by any expedients commonly used in the formation of joints in machinery, but by certain tough, fibrous, and elastic bands inserted firmly in the surfaces of the parts to be united ; in such a manner, nevertheless, as to allow free play to the moving parts within the prescribed limits. These bands are called ligaments, from the Latin word ligo, I tie. It will, therefore, be easily understood why upon the removal or disso- lution of such means of connection the bones would fall asunder. If no other expedient, however, were provided in the me- chanism of the joints, it is easy to see that the body, subject to the incidents to which it must necessarily be exposed, would be gradually deteriorated and soon destroyed. The surfaces of the bones, connected as described above, would be pressed one against the other by the incumbent weight of all that part of the body situated above the joint ; and upon every motion of the joint the surfaces in contact would rub one against the other with a degree of friction proportionate to the force by which they are pressed together ; and no matter what might be the character of their surfaces, such friction would gradually abrade and wear away the bone. 10. But independently of this, the connecting ligaments being more or less elastic, the contiguous surfaces united by them are not always in contact, and are more or less separated according as, by the varying attitude of the body, the parts united are released from the pressure which urges them together. It would consequently happen that, in the frequent incidents of bodily motion in which the members are subject to slight shocks or concussions, as, for example, in rapid walking, run- ning, or still more, in vaulting or leaping, the joints would rattle, the contiguous surfaces impinging on each other, as hard bodies do when they accidentally come into collision. Besides other injurious effects which are sufficiently obvious, the surfaces of the bones at the joints would be chipped and cracked, and the mechanism of the system soon utterly destroyed. 11. Let us consider for a moment how similar injurious effects are obviated in mechanical contrivances. If the carriages of a railway train, for example, were con- nected one with another by chains or cords more or less flexible, they would enter into violent collision one with another, not only when the train would suddenly stop, but on GENERAL VIEW. 7 the occasion of every sudden change of speed ; the vehicles thus striking against each other, the parts subject to such collision -would soon be broken and destroyed. All objects transported by the vehicles would share, more or less, in these shocks consequent upon every change of momentum, and for the transport of passengers the carriages would thus be rendered intolerable. Now, these evils are effectually prevented by attaching to the ends of the carriages soft cushions, called buffers, behind which are springs to give them increased elas- ticity. The shocks produced by the causes above mentioned are broken and mitigated so as to be rendered scarcely perceptible, save in extreme cases, by these expedients — the momentum of the shock being expended upon the cushion and spring. A similar expedient is applied in the skeleton. A coating or cushion of a tough, fibrous, and elastic substance called cartilage, interposed between the bones at the joints, breaks the force of collision in precisely the same manner as do the buffers established between the successive vehicles composing a railway train. 12. Interosseous Cartilage. — In some cases where the two bones jointed together are not intended to move in contact with each other, but only to change their relative position within small limits, so as to give a certain flexibility to their combination, the surfaces connected are flat, or nearly so, — equal in magnitude, similar in form and parallel. The cushion of cartilage interposed, is firmly attached to both of them, so that the surfaces which it unites admit of no lateral or sliding motion. In this case the interosseous cartilage serves a double purpose. Its softness and elasticity break the force of collisions, and its flexibility allows the two bones it connects to be inclined to each other in any direction within certain limits, such inclination being attended with a compression of the inter- posed cartilage on the side towards which the inclination takes place, and an extension on the contrary side. It is evident that the degree of flexibility, permitted by such a joint, will vary with the thickness of the stratum of interposed cartilage. The cases, however, are much more numerous in which a greater play being allowed to the bones, their surfaces must move with a sliding motion one upon the other. The mechanical conditions, necessary to give efficiency to such a joint and per- manency to its action, are much more numerous and complicated. As in the class of joints above described, a qpating of inter- osseous cartilage is provided to break the force of collisions. 8 ANIMAL PHYSICS. But as the mere elasticity and flexibility of such cartilage would be altogether insufficient to allow of the extensive play required to be given to the bones, the surfaces must move freely one upon the other, and consequently the interposed cartilage cannot be as in the former case a single stratum with its oppo- site surfaces adhering to the bones. Two separate and independent strata, or coatings, are therefore provided in the.se cases — one attached to each osseous surface. These completely intercept the contact of the bones, and the surfaces which really move upon each other are those of the cartilaginous coating, and not those of the bones. 13. Friction of Joints. — As the surfaces moving thus in contact are subject to a constant change of relative position, it is obviously necessary, not only that one surface should corre- spond in form with the other in one position, but that they should do so in all the positions which they are capable of assuming. This is a condition which, according to the principles of geometry, can only be fulfilled by giving to the surfaces a form which is either plane, cylindrical, or spherical, since the only lines which can move upon one another, so that every point of them shall be in mutual contact, whatever be their position, are the straight line and the circle. Two plane surfaces, therefore, can slide evenly one upon the other. Two cylindrical surfaces, one of which is convex and the other concave, can also slide evenly upon each other, all their parts being constantly in mutual contact, provided they have the same radius and a com- mon axis, and provided the motion take place round the axis of the cylinder, and not otherwise. Two spherical surfaces, one of which is convex and the other concave, can, in like manner, slide evenly upon each other, every point of the one being in contact with the other, provided that they have the same radius and a common centre, and pro- vided that the motion take place only round that centre, what- ever may otherwise be its direction. It will be seen hereafter, that these conditions are fulfilled in each case by all the joints in the animal economy which are composed of surfaces moving one upon the other. Although these conditions, however, provide against the in- jurious effects of collision, it remains to show how those which would attend the mutual friction of the cartilaginous coatings are prevented. In the first place, these coatings are so consti- tuted as to be susceptible of smooth and even highly polished GENERAL VIEW. 9 surfaces. Where the surfaces of the bones are plane and par- allel, the coatings are of uniform thickness ; but, where the surfaces are concave and convex, the coating of the concave surface is thinner towards the centre than towards the borders ; while, on the contrary, the coating of the convex surface is thinner towards the borders than towards the centre ; the result of which is, that in the normal position of the bones the sum of the thicknesses of the two coatings is uniform. 14. Adhesion of Joints.— It is not enough, however, that these interosseous surfaces should be endowed with the highest degree of smoothness, polish, and uniformity of figure. The very perfection of these qualities, combined with the force with which the joints are in many cases loaded, would even in the absence of friction produce a degree of adhesion between the surfaces, similar to that which takes place between two surfaces of polished metal which are brought into close contact ; an effect familiar to all workmen in metal, and called the bite. In short, this adhesion would not only obstruct, but in most cases completely arrest the motion of the bones thus connected, though all the prescribed conditions should be fulfilled in the strictest manner. 15. Synovia, its Uses. — In the practical construction of machinery, the obstruction arising from adhesion is removed by the lubrication of the surfaces which form the joint by some suitable liquid or semi-liquid substance. A similar expe- dient is accordingly applied for the same purpose in the animal economy. Every joint in which the surfaces move upon each other is provided with an apparatus by which a certain viscid glairy liquid is secreted, resembling in its physical qualities the white of egg, and hence called synovia. This liquid is constantly poured over the surfaces of the cartilaginous coatings, so as to produce the same effect upon them as oil does between the axle and the box of a carriage- wheel. In fact, this liquid, strictly speaking, by its interposition between the cartilaginous surfaces prevents their mutual contact ; and its molecules, being infinitely small and infinitely smooth spherules, have the effect of the most perfect imaginable friction rollers. By this admi- rable provision, therefore, the obstructions which would arise from both friction and adhesion are removed, the interposition of the polished rolling molecules removing those of friction, and the prevention of contact removing the possibility of adhesion. 10 ANIMAL PHYSICS. Anatomists aro not completely agreed as to the nature of the apparatus by which this lubricating fluid is secreted ; but they have designated it generally by the name of the synovial membrane. The beneficial effects of synovia are not, however, confined to the joints, but extend generally to every part of the system, where surfaces move upon each other ; and, as in the change of position of the bones, the ligaments which connect them at the joint must move, more or less, upon the surface of the bone, their surfaces are likewise lubricated by the synovia. The synovial membrane surrounds the joint on every sile, and thus contributes in the most effectual manner to maintain the bones in contact, by excluding from the space between them all surrounding fluids which would be exposed to the atmos- pheric pressure. The consequence of this is, that any attempt to separate the bones at the joint would necessarily be resisted by the whole force of the atmosphere. To be assured of this, it is only necessary to attempt to disjoin the bones of a dead subject ; the resistance will be found to be considerable. But if the synovial membrane surrounding the joint be punctured, so as to admit the air, the dismemberment will be comparatively easy. 16. Muscles. — The apparatus by which the bones are held together being described, it remains to show how those move- ments of which they are severally susceptible are imparted to them. The bones themselves are merely passive instruments ; and the ligaments by which they are connected, the forms given to them at the joints, the cartilaginous coatings, and synovial apparatus, are provided respectively for facilitating but not at all for originating their motions. The apparatus by which the motions are immediately pro- duced, are fibrous bands and masses of flesh called muscles, which constitute that part of the animal body which when used for human food is called meat. With the visible fibrous struc- ture of the muscular tissue every one must be familiar. Muscles consist of fibres ranged generally side by side, parallel to each other. They are extended between the bones, to one or both of which they are intended to impart motion ; or, as in the face and eye, one end only is attached to bone. The muscle itself, however, is not immediately connected with the bone. At its extremities it gradually takes the form of tendinous fibres, totally different in their physical character from the fibres of the muscle itself. GENERAL VIEW. 11 17. Tendons. — These tendinous fibres axe sometimes collected into a single cord called a tendon, which is inserted in the bone so firmly that before it can be detached from it the bone itself would be broken. Sometimes their extremities are spread out on a line of greater or less length ; and, instead of being inserted at a single point of the bone, are attached to it along a line of corresponding extent. In such cases, the tendinous connec- tions of the muscles with the bone are called aponeuroses. 18. When two bones are connected by a muscle, it generally happens that the normal action of the muscle is to impart motion to one only of the two bones. In that case, the bone which it moves is to be regarded as a lever, — the point where the tendon of the muscle is inserted being the point of application of the power, and the point where the two bones are united being the fulcrum. It is customary to denominate the point where the tendon of the muscle is attached to the bone to be moved, the insertion, and the point where the other tendon is attached to the connected bone, the origin of the muscle. This distinction, however, cannot always be rigorously observed, inasmuch as in some cases the muscle acts indifferently iu imparting motion to either bone. 19. The property by which the muscles move the bones is a power of contraction, which constitutes their peculiar and distinguishing character, and in which no other parts of the animal organisation participate. By this power they are enabled to diminish their dimensions measured in the direction of their fibres ; and, since these fibres are extended between the origin and the insertion, it follows that by such a contrac- tion the tendon of the insertion is drawn towards the tendon of the origin, and with it necessarily the bone in which the insertion is made, is drawn towards that in which the tendon of the origin is implanted. This contractile power of the muscle may be compared to the tension of a spring, and the tendons to the straps by which the spring is connected with the object upon which it acts. Like the straps, the tendon or aponeurosis has no contractile power, and produces no effect whatever upon the motion of the bone. It is merely the instrument by the intervention of which the contractile elasticity of the muscle is conveyed to the bone, exactly as the elasticity of the springs upon which a carriage is suspended is conveyed to the body by the straps which connect the one with the other. 12 ANIMAL PHYSICS. The contractile force of the muscles has no reaction. There is no corresponding extensile force. When by the contraction of a muscle one bone has been drawn towards another, it will be held in that position so long as the contraction continues. But as this contraction absorbs a certain amount of animal force, and as this absorption is ccderis paribus in the exact proportion of the continuance, it would necessarily happen that after a certain interval the animal energy winch produces the contraction would relax, and the bone upon wThich the contrac- tion had acted being liberated from the force to which it was previously subject, would be free to move in obedience to any other force which might act upon it. But, if no other force acted upon it, the relaxed muscle would have no power to move it back to its original position. It follows, therefore, that in all cases in which a bone is moved alternately in contrary directions, the action of two muscles placed on opposite sides of it is necessary. 20. The two muscles which thus exert opposite actions are said to be antagonistic. It is evident from what has been stated, that the points of origin and insertion of two antagonistic muscles must be placed on opposite sides of the bone on which they act. Since a muscle can impart no other motion to a bone than that which is determined by the relative positions of its points of origin and insertion, it follows that a bone which is suscep- tible of many different motions with relation to the bone with which it is connected, must be moved by the action of as many different muscles. When, as frequently happens, two different muscles conspire to impart a certain motion to a bone, they are said to be congeneratc. It has been ascertained that when a muscle is contracted between its origin and insertion, it undergoes no real diminu- tion of volume, since it suffers an increase of dimension in a direction transverse to that of its contraction, which is com- mensurate with the decrease of its dimension in the latter direction. Any one can comince himself of this by observing the effects produced by the contraction of the muscles of his own limbs. If the hand be raised to the shoulder so as to bend the elbow-joint to an acute angle, the muscle producing this motion, winch has its insertion in the fore-arm and its origin in the arm, will be gathered into a voluminous lump on the a.nn between the shoulder and the angle of the elbow, a GENERAL VIEW. 13 lump whicli can be distinctly seen and felt, and which presents a remarkable appearance in persons of strong muscular develop- ment. 21. Muscular Force. — Anatomists and physiologists have not determined with certainty the mechanical change by which muscular contraction is produced. When the muscular tissue is submitted to a microscope of moderate magnifying power, one, for example, of five or six times the linear dimensions, each fibre is found to consist of a number of fasciculi, each similar to the original fibre. In fig. 1, a represents a small portion of a muscle in its natural size, cut transversely at its extremity. n represents the same object magnified five times in its linear dimensions, the component fibres of which it consists being rendered apparent. Fig. 2 represents a part of a muscle sub- mitted to a much higher magnifying power, in which the structure of each separate fibre is shown as marked by a series of transverse stria). The terminal section is shown at a a, the transverse striae at b l, and a single fibre split into its com- ponent fibril la; at c. When muscles have been examined with the microscope in the process of contracting, their transverse striae have been ob- served to approach each other, an effect which would necessarily be accompanied by a corresponding diminution of their dimen- sions in the direction of their fibres. 14 ANIMAL PHYSICS. The contractile power of the muscles which hare been described can, in general, only be called into action by the dictate of the will. Hence they are called voluntary muscles, and examples of them are presented by the muscles which impart motion to the principal members of the body. Thus, the muscles by which the legs or arms are moved, can only be brought into play by the operation of the will. There are some muscles which are, to a certain extent, subject to the will, but also act independently of it. The muscles which move the chest in respiration present examples of this class. The will has the power of accelerating, retarding, or even of tem- porarily suspending the act of respiration ; but when the will exercises no influence on the organs of respiration, as when the mind is engaged in other objects, or in sleep, the process of respiration goes on with perfect regularity. 22. Involuntary Muscles. — There are some muscles over which the will has no control whatever, and which are hence called involuntary muscles. The heart, and themuscles entering into the structure of the stomach and intestines, are examples of this class. Except the heart, they do not present striae. The involuntary muscles, and those of a mixed character, like the voluntary muscles, absorb a certain amount of animal energy by their contraction, and consequently such contraction could not be maintained continuously without exhausting the animal power. We find, accordingly, that nature has so regu- lated the organisation, that all muscular action which is inde- pendent of the will is intermitting, so that the intervals of muscular repose or relaxation are, on the whole, equal to those of muscular tension. The heart of an animal beats incessantly, sleeping or waking, during the continuance of its vitality ; and this action may continue in man even for a century. The muscles, however, which produce it are never in a state of tension for more than a moment, so that they are enabled to recover their energy in the alternate intervals of their relaxation. 23. Blood. — The fluid by which this system of bone and flesh is nom-ished is the blood, which, having its fountain in the heart, is propelled from thence by the strong muscular action of that organ through a system of flexible tubes called arteries, which, like the trunk and branches of a tree, are of large calibro at their point of origin, and grow gradually less as they GENERAL VIEW. 15 ramify through the system, until they terminate in another set of pipes called, from their extreme minuteness, capillaries. From thence the nourishing fluid passes into another system of tubes, called the veins, through which it is carried back to the heart. The form and distribution of the veins is something like that of the arteries, their trunks entering the heart, and their minute ramifications being connected with the capillaries. While the blood, however, passes in the arteries from the trunks to the branches, it passes in the veins from the branches to the trunks. In passing from the arteries to the veins through the capillaries, the blood undergoes a remarkable change in its physical qualities. Having given up its nutritious elements to the organs through which the capillaries conduct it, it is carried back by the veins to the heart, to receive a fresh supply of the nutritive constituents. Its colour is also visibly changed, the arterial blood being bright red, and venous blood blackish red. 24. Lymph and Chyle. — Another system of tubes originat- ing in all parts of the body consists, like the veins and arteries, of ramifications and trunks conducting from every part of the body, but more especially from the intestines, to the heart, a fluid which in some parts is colourless and in others whitish. This fluid is called lymph, and the vessels which thus conduct it are called lymphatics ; that which is taken up from the intestines has the name of chyle. As in the veins, the lymph flows from the branches of the lymphatics to the trunks. These trunks discharge their contents into the venous trunks at points near those at which the latter enter the heart ; so that after the confluence of the lymphatics with the veins, the contents of the latter are a mixture of venous blood and lymph. This lymph contains a part of the nutritive elements by which the venous blood is renovated. 25. Circulation. — After this mixture of blood and lymph is discharged into the cavities of the heart by the venous trunks, it is again propelled to the lungs from the heart by the mus- cular force of that organ though a system of flexible pipes, called the pulmonary arteries. In the lungs this fluid mixture of blood and lymph is acted upon by the air received into the latter organs by respiration ; and here it undergoes the final change by which it recovers all its nutritive qualities, and is reconverted into bright red arterial blood. 16 ANIMAL PHYSICS. 26. From the lungs, after undergoing this change, this blood is propelled through another set of pipes called the pulmonary veins back to the heart, where it is received into another cavity, from which it is driven as before through the arteries into the capillaries, and back to the heart through the veins. Such are the phenomena which constitute what is called the circulation of the blood. 27. Respiration is a function intimately connected with cir- culation. The atmospheric air drawn into the lungs in breath- ing penetrates into the air cells of these organs ; and there acting on the blood through the tissues, some changes of great vital importance take place. The oxygen of the atmosphere is absorbed, and the carbonic acid, with which the venous blood was charged, is liberated. This double effect produces the final change by which the blood recovers its arterial character. The air we expire, consequently, in respiration, being deprived of the chief part of its oxygen, is charged with a certain quantity of carbonic acid. 28. Circulation and respiration are therefore the phenomena which constitute the proximate source of nutrition. The arte- rial blood, passing through the system, deposits in every part of it the nutritive principles, and receives in exchange a portion of what the body rejects. The lymphatics, collecting the nutri- tive lymph from various parts of the system, pour it into the venous blood, with which it is carried to the lungs, where the venous blood gives up the noxious elements which the organs had rejected and thrown into it, these noxious elements being expired in respiration, and where it receives the oxygen of the atmosphere, which is necessary, combined with the lymph, to reconstitute its nutritive power. 29. Nerves. — In assigning the muscular apparatus and its contractile power as the proximate agency to which the motions of the body are to be ascribed, we have advanced one step, but only one, to the origin of these motions. What, it may be asked, induces those muscular contractions which are them- selves the cause of the bodily motions ? What conveys the dictates of the will with such promptitude and precision at each moment, to any one, or to many at once, of several hundred muscles distributed throughout the body ? This wonderful effect is produced by the still more wonderful apparatus of the nerves. These complicated threads, originating GENERAL VIEW. 17 I in the brain, diverge in thousands of directions from that as a centre to every part of the system. Their main trunk, pro- ceeding from the back of the skull down a central perforation in the backbone, throws out on the one side and the other, through lateral orifices, innumerable ramifications, which ex- tend to every part of the body. 30. Each muscle receives one or several nerves, each of which is enclosed in a sheath or covering called a neurilemma. These nervous filaments, thus enclosed, traverse every part of the muscle in directions parallel to each other, and perpen- dicular to the muscular fibres. The greater part of them pass on to other muscles, and are inoperative relatively to that which gives them passage. But some terminate in the muscle, or, according to the opinion of some physiologists, after looping themselves upon the muscular fibres, they rejoin the filaments by which they arrived at the muscle, and thus return to the brain. 31. Brain. — The brain, the agency of thought and volition, exercises upon the origin of the nerves a certain power, which may be aptly enough illustrated by the power which a voltaic battery exerts upon a conducting wire ; and by this action, some subtle influence is transmitted along the nervous cord to the muscle, where it terminates, or on the fibres of which, as explained above, it is looped ; and this influence, acting in a specific manner, causes the contraction, which imparts motion to the member with which the muscle is connected. It is impossible to read this simple statement without being struck with the analogy which prevails between the nervous system and a voltaic arrangement. If, as is contended, each nervous cord, upon arriving at the muscle upon which it acts, returns to the brain, nothing is wanting to complete the analogy to the electric telegraph. The brain is the telegraphic instru- ment receiving the dictates of the will. The nervous cord is the conducting wire which, arriving at the remote station — that is, the muscle — returns to the brain to complete the voltaic circuit. The message delivered to the muscle produces the commanded motion, just as the voltaic current imparts motion to the signal needle at the distant station. That the nervous cord is in fact the conductor of the physical influence, whatever it may be, transmitted by the will from the brain, is proved by the fact that if the nervo be cut anywhere between the muscle and the brain, all power of the will over o 18 ANIMAL PHYSICS. the muscle ceases, and the member to which the muscle imparts motion becomes paralysed. It is not even necessary to cut the nerve to produce this effect. If the brain at the origin of the nerve be submitted to a certain compression, its power of transmitting the influence, whatever it may be, to the muscle, will be suspended, and will only be restored when the com- pression is removed. 32. Among the physical researches in which physiologists have been from time to time engaged, with a view to discover the nature of the influence which the will thus transmits from the brain to the muscles, those which have obtained by far the greatest celebrity — not so much for the physiological consequences, as for the vast and important discoveries which have resulted from them — are those of Galvani, Professor of Anatomy at Bologna, from whom the branch of physics called “galvanism” has taken its name. It is found that the nervous cords, like metallic wires, are good conductors of electricity, and that if, after paralysing a member by cutting the nerve which connects its muscle with the brain, the extremity of the nerve leading to the muscle be put in connection with a voltaic battery, the muscle will be contracted and the member will be moved in the same manner as it would be by the action of the will. In like manner, if after death, when thought and will have ceased, the nerves which connect the brain with the several muscles which govern the members and the organs be put in connection with a voltaic battery, motions will be produced precisely the same as if thought and will had been restored to the sub- ject. Galvani’s original experiment upon the limbs of a frog is well known, and has been often repeated. Bailey substituted a grasshopper for a frog, and obtained similar results. 33. Like effects have been produced upon the human body recently deprived of life. Aldini in this way imparted violent action to the various members of a body ; the legs and feet were moved violently, the eyes opened and closed, and the mouth, cheeks, and all the features of the face were agitated by distortions. Dr. Uro connected one of the poles of a battery with tho supra-orbital nerve of a man cut down after hanging for an hour, and connected the other pole with the nerves of the heel. Each time the circuit was completed and broken, the limbs and features were moved with a fearful activity ; rage, anguish, despair, and horrid smiles were successively GENERAL VIEW. 19 expressed by the countenance, with the most revolting resem- blance to actual vitality. 34. Nerves of Motion and Sensation. — The nerves, how- ever, have another function equal in physiological importance to that in virtue of which they impart motion to the organs and the members. They are the conductors by which impres- sions produced in all parts of the system, whether by external or internal causes, are transmitted to the brain, and by which the corresponding sensation is produced. When we behold a visible object, the impression produced upon the eye is trans- mitted by the optic nerve to the brain, and we obtain a percep- tion of the object. "When the vibrations of the air produced by a sounding body act upon the ear, or when the effluvia of a rose act upon the olfactory organ, the auditory or the olfactory nerve transmits the impression to the brain, and we are con- scious of the sensation of the sound or of the odour. If we touch a body which is rough or smooth, sharp or blunt, hot or cold, tho nerves which are near the point of contact will be affected in a peculiar manner by the texture, form, or temperature of the body, and will, as before, transmit to the brain corresponding impressions, which will produce the sensations by which we acquire the perception of the qualities of the body thus touched. Thus it appears that the nerves include not only the me- chanism of mobility, but that of sensibility ; and the researches of physiologists have conducted them to the veiy curious and interesting discovery, that the nerves which constitute the mechanism of sensibility are altogether distinct from those which constitute the mechanism of mobility. The latter have been accordingly called the nerves of motion, and the former the nerves of sensation. Notwithstanding the complete independence of these two nervous systems, so far as relates to their respective functions, they are often united mechanically together, so that their fibres form a single nervous cord. In such cases, however, they frequently diverge one from another, like the branches of the letter Y, before arriving at the brain, so that one branch includes only the fibres of the nerve of motion, and the other only those of the nerve of sensation. If, in such cases, the latter branch be cut, the organ to which the nerve is appro- priated loses its sensibility, but retains all its mobility. Tho will has still complete control over it, but the organ becomes insensible to external impressions. c 2 20 ANIMAL PHYSICS. If, on the contrary, the other branch be cut, the organ retains its sensibility, but becomes paralysed. If, in fine, both branches be cut, the organ is paralysed, and also rendered insensible. 35. Digestive Apparatus. — The nutritive matter supplied by the lymphatics to the blood is eliminated from the food by an apparatus called the alimentary canal, consisting of the stomach, the intestines, the liver, pancreas, and other appendages. Prom the mouth, a pipe called the oesophagus conducts the food to a bag called the stomach, deposited within the cavity of the abdomen, immediately below the heart and lungs. From this cavity a flexible pipe called the intestine, measuring many feet in length, proceeds, and being coiled up is packed into the lower part of the abdomen. Into this pipe the liver sends one communication, and the pancreas another. The food undergoes a succession of changes, the first of which is effected in the stomach ; and in its course there are mixed with it certain juices from the liver, the pancreas, and the coats of the intestine. The nutritive principle is gradually eliminated from the food in its progress through the stomach and intestines ; and this prin- ciple, by a species of capillary action called exosmose, penetrates the coats of the stomach and intestine, and is received into the countless minute ramifications of the lymphatic system with which they are surrounded. The white blood, or lymph as it is called, is thus supplied to these parts of the lymphatics, and transmitted thence, as already described, to the venous trunks. 36. Having thus briefly described the organisation of the body, we shall resume each of the principal subjects here indi- cated, explaining them with all the detail necessary to render their principles understood ; after which the several organs of sense will be described. THE BONES AND LIGAMENTS. 21 CHAPTER II. THE BONES AND LIGAMENTS. 3 T. The framework by which the softer parts of the organ- isation are sustained, called the skeleton, consists, like the body, of three distinct parts — the head, the trunk, and the members. Trunk.— The trunk is a bony cage consisting, in the human species, of a jointed vertical pillar, called the vertebral column, extending from the lower extremity of the back to the base of the neck, to which are attached, laterally, a series of hoops united to a shorter bone in front, also vertical, and occupying the middle of the breast, called the breast-bone or sternum. The hoops just mentioned, being, therefore, divided behind by the vertebral column, and in front by the sternum, are resolved into two series of semi-circular, or rather semi-elliptical, pieces ranged one above the other at nearly equal distances, and in nearly parallel positions. These semicircular hoops are called the ribs, and they form an oval cage, the lateral diameter of which is greater than the antero-posterior, or that which passes from the sternum to the vertebral column. Within this cage the heart and lungs are included and protected. Upon the summit, or capital, of the vertebral column the head is mounted and supported. To the superior lateral comers of it are attached the superior or thoracic members, which, in man, are called the arms ; and to the inferior lateral comers are, in like manner, attached the inferior or abdominal members called the legs. 38. Number of the Bones. — Without taking into account some small detached bones, regarded as accessory to particular organs rather than properly belonging to the general frame- work of the system, the skeleton may be said to consist of 198 separate bones, which are, nevertheless, connected together in the manner already described, by means of ligaments and other cartilaginous appendages. The distribution of these bones in the system is as follows : — • 22 ANIMAL PHYSICS. Vertebral column .... . . . 26 Skull . 8 Face Hyoid-bone of the neck . 1 Ribs and Sternum .... . . 25 Arms (32 each) .... . G4 Legs (30 each) 108 We Lave liere used the terms “legs” and “arms” in the popular sense of the words, as including in the former all the hones of the legs and the feet, and in the latter all those of the arms and the hands. 39. Growth, of the Bones. — The enumeration of the bones is not so obvious a problem as it might at first appear, inas- much as the number of separate and independent bones is not the same at different ages. Thus, the bones are more nume- rous in infancy than in youth— in youth than in manhood — and in manhood than in old age. In the first stage of intra- uterine existence, the bones are composed exclusively, first of mucus, and later of cartilage. In successive stages of growth they receive accessions of other constituents, which give them that hardness that constitutes the osseous character. Ossi- fication gi'adually spreads with time, and bones which in earlier stages were independent, or only connected together by carti- lage, are united so as to form single osseous pieces by the ossification of the connecting cartilage. In enumerating the bones, therefore, it is necessary to specify the epoch of life to which the proposed enumeration is applied. That which is given above is accordingly applicable to the age of from twenty- five to thirty, when the development of the organisation may be considered as most complete. 40. Constituents of the Bones. — When it is considered that the purpose of the bones is to give solidity and strength to the frame — not merely for the support and motion of their own weight and that of the other parts of the body, but also to resist those external forces to which the body in the common incidents of life is exposed, and which •would tend more or less to derange or fracture them, — it may be anticipated that in accordance with that spirit of unbounded wisdom and bene- ficence which is observable in all the works of Nature, and in none more so than in the animal organisation, the constitution THE BONES AND LIGAMENTS. 23 and structure of the bones should be such as would resist all these causes of ordinary disturbance. They must be firm to resist pressure, tough to resist extension, and more or less elastic to be capable of yielding to a limited extent without fracture. We find, accordingly, that their constituents are such as to confer on them precisely these three important qualities, and to impart them in that degree which is necessary and sufficient to protect them from all injury and derangement arising from disturbing causes of ordinary occurrence. The bones derive these properties from three constituents — fibre, cartilage, and certain species of earthy matter, the prin- cipal of which is phosphate of lime. From the fibre they derive toughness ; from the cartilage, elasticity ; and from the earthy matter, hardness and firmness. 41. In comparing one with another the different species of animals, these constituents are found to enter into the compo- sition of their bones in different proportions, each constituent predominating according as one or other of the mechanical qualities they confer are most necessary to the habits and economy of the animal. Thus, in the case of fishes which inhabit an element that supports their weight, the bones having no need of the strength of pillars, but, on the contrary, requiring great elasticity to give effect to the muscular action of the animal in propelling itself through the water, are formed with a very large proportion of the cartilaginous constituent, and in certain species are so nearly destitute of the calcareous element, that they have received the name of cartilaginous fishes. On comparing one with another the different species of land animals, the same beneficent adaptation is everywhere observable. Those animals whose habits and economy require most elasticity in their solid structure, have a greater proportion of cartilage ; those which require most toughness have bones with a predo- minance of fibre ; and those which require greatest solidity and firmness have a larger proportion of the calcareous constituent. 42. Not only is the nice adaptation of the proportion of the constituents of the bones to the necessities of animal life observable in species compared with species, but it is even found to prevail in the same animal compared with itself at different ages, and in different parts of the organisation at any given age. The helpless infant, exposed by a thousand incidents to ex- ternal shocks, has a skeleton, a considerable part of wliich, being 24 ANIMAL PHYSICS. gristly and cartilaginous, is yielding and elastic, and therefore incurs little danger of injury. The youth, whose augmented weight and increased activity demand greater strength, has a larger proportion of the calcareous and fibrous elements, but still sufficient of the cartilaginous to give the necessary combi- nation of firmness, toughness, and elasticity. As age advances, and prudence and tranquil habits increase, as well as the weight which the skeleton has to sustain, the calcareous element increases and the cartilaginous diminishes. 43. In comparing bone with bone at any given age, the same beautiful adaptation is observable, each having just that proportion of earth, cartilage, and fibre which is best suited to its functions. Thus, for example, the temporal bone, in which the organ of hearing is mounted, is as dense and hard as marble, whence it has been called the os petrosvm. It is there- fore eminently suited, by its vibratory character, to propagate to the auditory nerves the undulations of the air. The bones of the heel and elbow, on the other hand, which are subject to the constant action of powerful muscles — which, in the case of the heel, have to react upon the entire weight of the body — require all the toughness of a rope, and are accord- ingly found to contain a predominance of the fibrous element. In fine, the columnar bones of the leg, which form pillars upon which the weight of the body is sustained, require and possess a corresponding excess of the earthy element.* 44. That the bones do actually consist of such a combi- nation of earthy and gristly matter as has been described above, can be ascertained by two simple and easily executed experi- ments, the result of one of which is the exhibition of the gristly matter separate from the earthy ; and of the other, the exhi- bition of the earthy separate from the gristly. If a bone be steeped in dilute nitric or hydrochloric acid, the earthy- constituent being dissolved by the acid will be extricated ; the tough, flexible, and gristly substance, which will not be affected by the acid, remaining. If, on the other hand, the bone be burnt in an open fire, with free access of air, the fibrous and gristly part will be first charred, and after- wards will undergo the process of combustion. The earthy matter alone will remain, having a white, brittle consistency, still preserving its original shape, the bone having, however, lost about a third of its weight. On examining this earthy residuum, it is found to con- sist principally of a chemical substance called phosphate of lime, — being, as the name indicates, a salt compounded of phosphoric acid Bell on the Hand, 6th Edition, p. 301. THE BONES AND LIGAMENTS. 25 and lime. Other earthy substances are combined with it, but in smaller proportions. The exact analysis of bone, according to Berzelius, whose results were confirmed by the experiments of Mr. Middleton, made in the laboratory of University College, London, is as follows : Berzelius. Middleton. Animal matter .... 33 '30 — 33 '43 Phosphate of lime . . . . 51 '04 — 51 'll Carbonate of lime ..... 11'30 — 10'31 Fluoride of calcium . . . . . 2 '00 — 1'99 Magnesia, wholly or partially in the j i .i c i .<57 state of phosphate . . . . ) Soda, and chloride of sodium . . . 1'20 — 1'68 100-00 100-19 These proportions, however, as has been already stated, are subject to some variation. 45. In the preceding paragraphs the variation of the constituents of the bones at different ages has been assumed, as it has been generally admitted hitherto by anatomists. Some recent experiments by M. Nelaton have called in doubt these facts. That anatomist maintains that he has found the proportion which prevails between the organic and calcareous elements of the bones to be the same at all periods of life, and that the bony tissue is not a mere mechanical mixture of gelatine with the calcareous con- stituent, but that it is a chemical combination, into which these elements enter in definite proportions. The experiments of M. Nelaton were repeated in concert with M. Sappey, when the same results were obtained ; the average proportion of the organic to the inorganic elements at all periods of life, from the moment of birth to the most advanced old age, being found to be that of 32 to 68. These results, nevertheless, do not affect the moral argument, based upon the varying mechanical qualities of the bones at different periods of life, adapting themselves to the varying circumstances in which the indi- vidual is placed. The fact that these qualities of the bones do so vary, is not disputed by Messrs. Nelaton and Sappey, who only deny that this variation can be explained by any change in the proportion of the constituents. They admit the gradually increasing density, decreasing vitality, and increasing rigidity of the bony tissue with increasing age, but propose other hypotheses for its explanation. 46. The mechanical properties of the bones depend upon their structure and their form, as well as on their constituents. The same adaptation to their functions will accordingly be found as well in the former as in the latter. On sawing through a bone at right angles to its surface, so as to exhibit by section its internal structure, it will be found to consist externally of parts which are dense and close in texture, having a resemblance to ivory ; and internally, of parts whose texture is open, reticular, and cellular. Anato- mists have accordingly denominated the former the compact ; 26 ANIMAL PHYSICS. and the latter, the cancellated tissue, from the Latin word “ cancelli,” signifying lattice- work. Such a combination of dense exterior texture and light porous interior structure is precisely that which, according to mechanical principles, must produce the greatest amount of strength combined with the least amount of weight. 47. The form of the bones is infinitely various ; and what- ever classification of them may be attempted, must be more or less arbitrary and vague. That which has been generally received and found practically convenient, though subject to some objections, resolves the parts of the skeleton into : 1°. Long bones, which are those of which the thickness or diameter bears a small proportion to the length ; the principal bones of the legs and arms are examples of these. 2°. Short bones, which are those whose thickness or diameter does not differ much from their length ; the small bones of the wrist and ankle are examples of these. 3°. Tabular bones, or, as they are sometimes though improperly called, flat bones, are those of which the thickness bears a very small pro- portion to the length and breadth. The surfaces of these are generally more or less curved ; one being convex, and the other concave. The bones which form the roof and sides of the skull are examples of these. 4°. Irregularly formed bones, not very properly so denominated, are such as cannot be brought within any of the preceding classes. Such bones generally occupy the middle part of the body, so as to be intersected by the median plane, which in general divides them symmetrically. The vertebra, and certain internal bones of the head, present examples of this class. 48. The median plane here referred to is an imaginary plane passing through the centre of the body in a vertical direction, from the middle of the front to the middle of the back, dividing the body into two parts, the right and the left, perfectly similar and symmetrical. This plane is of great use in defining the position of different organs of the body, and will be frequently referred to for that purpose. 49. The long bones, owing to the length of the lever upon which any strain upon them must act, require to be constructed upon those principles which will confer upon them the greatest amount of strength which a given weight of material can receive. According to the principles of mechanics, their forms should be that of a hollow tube ; such is accordingly found to be the case : and moreover, the mechanical principle is carried THE BONES AND LIGAMENTS. 27 to its extreme limit, inasmuch as the material of the tube gradually increases in density in proceeding outwards from the internal to the external surface, the internal parts being com- posed of cancellated, and the external of compact tissue. The transition from the one tissue to t'he other is not sudden and definite, but gradual and imperceptible, no distinct limit being discoverable fixing the point at which the cancellated tissue ends and the compact tissue begins. The difference between these bony tissues is one, therefore, of degree, and not of kind. They are both porous, but porous in very different degrees ; the porosity of one being visible to the naked eye, while that of the other is microscopic. 50. Short Bones. — The strains to which these are subject are considerably less, and act upon them with a much less leverage ; there is, therefore, less necessity for pushing the mechanical principle which confers strength upon them to its extreme limit ; and it is accordingly found that their internal texture is everywhere spongy, the compact tissue being limited to a thin superficial crust which invests them. 51. Tabular bones are composed of two nearly parallel shells of compact tissue, the intermediate space being filled by cancellated tissue, called diploe. This combination, besides the general pur- pose of augmenting the strength conferred upon a given quan- tity of the material, is attended with the further advantage of intercepting in a greater or less degree the vibrations produced by shocks and concussions proceeding from external causes, and thus acting as a sort of damper or buffer for the preserva- tion of the body. This advantage deserves a special notice in the case of the skull, which, as has been already explained, includes within it the important and delicate organisation which constitutes the centre of thought, sensation, and volun- tary motion. A concussion upon the bony casing smTOunding this, would, if not intercepted, derange or even paralyse the most important of the vital functions. The spongy tissue, therefore, interposed between the inner and outer shells of the compact tissue, is hero of capital advantage ; it has the same effect precisely as has the padding which lines the warrior’s helmet or the huntsman’s cap ; it intercepts the force, and arrests the vibration of the blow. 52. The Hair. — Though not immediately connected with the subject of the bones, it may not be out of place here to 28 ANIMAL PHYSICS. remark one of the advantages which the hair covering the surface of the head supplies. As the intermediate spongy tissue in a certain degree intercepts a concussion received by the external tissue of the bone, the hair in like manner pro- tects that tissue itself, receiving and intercepting every external force ; and it may be added, that when in its natural state its growth is allowed, it falls upon the neck, and protects the upper vertebrae as well as the skull. It has been well observed that art, even in the rudest stages of society, takes nature for her model ; and we find the helmet of the ancient warrior crested with horsehair, descending to a certain point over the neck and shoulders, to give that protection to them, which the natural hair is intended to afford to the naked savage ; and this expedient of ancient times continues to prevail in the modem military costume. 53. The distribution and arrangement of the compact and spongy tissues in the irregular bones, is determined on like principles. That these two tissues differ in nothing save the degree of their porosity can be shown by slicing a bone at right angles to its length. The reticulated structure of the cancellated tissue will then be evident to the naked eye. On approaching the compact tissue, the pores and cells become gradually smaller, until they become altogether imperceptible. If the section, however, be submitted to examination with the micro- scope, it will present the appearance of a surface covered with numerous little round apertures, as shown in fig. 3, which are the sections of a multitude of small canals which traverse the bone longitudinally, and are called Haversian canals, from Havers, an eminent English ana- tomist, who first directed attention to them. These canals, which in the living bone give passage to a multitude of blood-vessels and Pigr- 3. nerves, vary from the two-hundredth to the two-thousandth of an inch in diameter. 54. In fig. 3 the section of the hone, which is the ulna deprived of its earthy constituent, is shown in its natural size. The small portion of it which in that figure is marked with dark shading, is shown in fig. 4, as it appears with a linear magnifying power of twenty l the Haversian canals now appearing as circular or oval openings, each surrounded by a series of concentric lamellm or fine plates ; other lamella', especially near the borders, being parallel to the surface of the bone. This section has been taken, by permission, from Dr. Sharpey’s General Anatomy. A similar section, somewhat different in its details, given me by Dr. Mandl, is shown in fig. 5. THE BONES AND LIGAMENTS. 29 In fig. 4 it will be observed that a multitude of small dark specks are Fig. 4. Fig. 5. 30 ANIMAL PHYSICS. seen Interspersed among the lamellae which snrround the Haversian canals. These were long supposed to be elementary particles of bone, and were accordingly called osseous corpuscles. The improvement of the microscope, however, led to the discovery of their true character. When viewed with a linear magnifying power of about 200, they are distinctly seen to be holes like the Haversian canal itself, but smaller and of a different form. A transverse section of the compact tissue of one of the bones of the arm, as viewed with a linear power of 150, is shown in fig. 6. What were formerly corpuscles, now called lacunce, here appear to form part of the concentric lamellae, having a form which has been compared to that of little Fig. 6. black insects. From the lamellae, innumerable minute canals traverse the intermediate space, connecting the lamella; with each other and with the central Haversian canal. This system of microscopic canaliculi obviously serves as passages for the circulation of the blood, and thereby for the distribution of the necessary nutritive element to produce the growth and repair the waste of the bone. 55. Periosteum. — This vascular apparatus of the bone is completed superficially by a membrane which covers it, called the periosteum , in which innumerable blood-vessels and nerves ramify and pass thence into the bone through its superficial pores. 56. If the bones were subject on all sides and in all direc- tions to equal strains and disturbances, their forms would be generally cylindrical or otherwise uniform, and the compact THE BONES AND LIGAMENTS. 31 tissue would have uniform arrangement and thickness. This, however, is obviously not the case in nature. Different bones, and different parts of the same bone, are subject to different external strains and disturbing forces ; and the form of its section and relative thickness, and arrangement of the compact tissue, will vary accordingly. It will be thickest on that side, and in that part, which is most exposed to strain and disturb- ance. In the case of the tabular bones, the necessary resisting force in particular directions is obtained without sacrificing lightness of structure, by placing ribs upon them, in the direction in which the strength is required ; and when the strength is required in all directions, two of these ribs are placed at right angles to each other, giving the strength of an arch in transverse directions. 57. The form of the body renders it generally necessary that the muscles should be disposed on the surface of the bones, their fibres, whose contractile force moves the bones, being parallel to and in almost immediate juxtaposition with them. Now it is a principle in Mechanics that the efficiency of a force which acts upon a lever is greatest when its direc- tion is at right angles to the lever, and decreases indefinitely as the obliquity of that direction is in- creased. If, then, between its origin and insertion, a muscle lies in con- tact with and parallel to a bone, it follows that the obliquity of its direction would be so extreme, that it would lose nearly all power to move the bone. Thus, if the joint on which the bone turns be at J, fig. 7, the muscle M lying in juxtaposition with jj' the bones and parallel to them, would exert very little power upon its point of insertion, i ; while if it had the position represented at m', fig. 8, it would then act at right angles to the bone upon the point i, and therefore with complete efficiency. But since such a position of the muscle is not compatible with other conditions of the economy, the inconvenience is removed by another expedient. The bones are usually enlarged in a considerable proportion at the joint ; a circumstance rendered necessary by other 32 ANIMAL PHYSICS. mechanical considerations. This enlargement forms what is and the point of insertion be placed near to it, the action of the muscle will be nearly perpendicular to the bone. 59. The points of insertion of the muscles which move the long bones, are generally at a distance from the joint which bears a small proportion to the entire length of the bone, and consequently the lever has the- character of a lever of the third kind, where the power acts to mechanical disadvantage. But in these cases rapidity and promptitude of motion are of incompa- rably greater importance than the exertion of great force ; and in proportion as the leverage afforded by the point of insertion of the muscle is small, the range of motion imparted to the bone by a given contraction of the muscle is increased. 60. In certain cases the leverage of the muscle is augmented in a greater or in a less degree by providing for its point of in- sertion, the extremity of a projection issuing from the bone at right angles to its surface, or nearly so ; such a projection is called a process or tubercle, and is variously denominated accord- ing to its form or length. Thus, if its form be tapering and its length considerable, it is called a spinous process. If it have less length, or a blunt or round extremity, it is called a tubercle ; and if it be shorter still, a tuberosity. "Whatever be its form, however, its combination with the bone, from which it issues, gives to the latter, in relation to the muscle inserted upon its extremity, the character of a rectangular lever. These emi- nences or projections from the surface of a bone, sometimes being very short and having a flat circular or roundish end, are destined for a different purpose in the mechanical economy. Two such surfaces upon adjacent bones, corresponding in form and magnitude, being brought together, form a joint. In such cases, the process is usually called an articular process, but some- times it is designated a condyle. 61. In the bones generally, and more especially in those that are tabular, there .are numerous cavities, depressions, and per- forations. The purpose of the perforations is to give passage to M called the head of the bone ; and as it always lies between the origin of the muscle and its insertion, the muscle must necessarily pass over it. I Fig. 9. 58. The effect of this, as will be evident from fig. 9, is to render the direction of the force of the muscle much less oblique, and consequently to give it greater power over the bone. If the en- largement at the head of the bone be considerable, ARTICULATIONS. 33 nerves, veins, or arteries, from the parts at one side to those at the other side of the bone. Such perforations are called foramina, the most remarkable of which is that whichds provided in the base of the skull for the passage of the medulla oblongata to the spinal cord, and which, as has already been stated, is called the foramen magnum. The base of the skull, however, is perforated with a great number of much smaller foramina, which give passage to the cranial nerves, and the vessels which maintain circulation in the brain. Depressions and cavities, varying extremely in form, magni- tude, and length, which prevail in the bones, are variously denominated fissures, fossae., grooves, furrows, notches, and so on. When they are curved and shallow they are called glenoid cavities ; but, when deeper, cotyloid cavities. The longitudinal depressions form usually the beds for nerves or blood-vessels ; and the spherical or cylindrical cavities, the sockets for joints. 62. Articulations.— The expedients by which the bony pieces of the skeleton are connected together are called, as already mentioned, articulations, and are reduced according to the limits they impose upon the mobility of the parts connected, to three classes ; 1st, Those which allow of no mobility, and are immovable joints. This articulation is called synarthrosis, from two Greek words — avv, sun, together, and apdpov, arthron, a joint. 2nd. Those which are movable, allowing a certain limited play to the parts connected. This articulation is called diarthrosis, from the Greek word — St«, dia, through. 3rd. Those which are nearly but not altogether immovable, affording no more play than such as may be allowed by the elasticity of their cartilaginous connection ; this articulation is called amphi- arthrosis, from the Greek word — ap.i, amphi, both, as par- taking, in some degree, of the character of both the former. 63. Examples of immovable joints will be found in the principal bones of which the skull is formed ; those of mov- able joints, in the case of the bones of all the members, and of the intermediate class in the bones of the vertebral column. The distinction between the mechanical functions of the ex- tremities of a muscle, designated as its origin and insertion, has been already noticed. These two points generally lie on the two bones jointed together at different sides of their articulation, the insertion being placed upon the bone to which the muscle is chiefly intended to impart motion ; and as the production of D 34 ANIMAL PHYSICS. motion usually proceeds from the central toward* the extreme parts, the muscles generally have their origin in the bones which are nearer to the trunk, and their insertion in those more remote from it. Thus, for example, the muscles which move the fingers occupy chiefly the palm of the hand, and the fore-arm ; those which move the fore-arm have their origin between the elbow and shoulder ; and those which move the upper bone of the arm have their origin on the shoulder and breast. Although the motion is thus usually imparted from the more central to the more extreme parts, the reverse takes place in certain exceptional cases, as for example, when the body is suspended by the hand, and elevated by the contraction of the brachial muscle. In that case, fig. 10, the insertions of these muscles in the fore-arm, and arm, become the fixed points ; the action of the contractile power being thrown upon the points in. the arm and breast, which are usually denominated their origin. In like maimer, when a gymnastic exhibitor sus- pends himself by the feet with the body downwards, and raises himself by the muscular force of the leg, the usual mechanical func- tions of the origin and in- sertion of the crural and femoral muscles are inter- changed. Having noticed thus gene- rally the mechanical provi- sions of the bones, we shall pass to a brief examination of the arrangement, con- nection, and chief mechanical properties of the principal divisions of the body ; the Fig. 10. head, the trunk, and the the chimpanzee. members ; which are repre- sented collectively in fig. 11, where the names of the prin- cipal bones are indicated. G4. The skull is a hollow shell of bone, of an oval spheroidal form, wider behind than before, occupying the upper and THE SKELETON, 35 Frontal Parietal. Orbit (of eye) Lower jaw Cervical vertebrae Scapula (shoulder* blade) Humerus (arm) Lumbar vertebrae Forearm / Ulna •' 1 ltadius Ca imus (wrist) Metacarpus Phalanges (finger- bones; Femur (thigh) Tibia Fibula Temporal. Clavicle (collar- bone). True Ribs. False Ribs. Ilium. Pelvis. Patella (knee-cap). Tarsus (instep). .Metatarsus (lower instep). Phalanges (toes) D 2 Fig. 11. HUMAN SKELETON. 3G ANIMAL PHYSICS. bone is presented tc view in tlie one, and the occipital in the other. hinder part of the head, and having below and in front of it the bones of the face. Besides the external bones of the skull which extend from the summit of the nose and eyebrows to the back of the neck, and from ear to ear, there is an internal base extending horizontally between the ears, and backwards from the eyebrows, to the back of the neck, which forms the flooring of the hollow, bony chamber, of which the convex sum- mit of the skull is the roof, and the sides, the walls. It is within this chamber that the brain is included. The bones composing the roof, sides, and floor of this cranial cavity are eight in number, all immovably jointed together by the first-class of articulation described above, and denominated syn- a/rtlvrosis. These eight bones are named as follows : — the frontal; occipital; sphenoid and ethmoid ; two parietal, and two temporal. The single bones be across the axis of the head, the median plane- dividing each of them into two parts perfectly similar, symme- trical, and similarly placed. The double bones, the parietal and temporal, are symmetrically placed at each side of the median plane. The relative position of these bones upon the head, so far as they are visible on the external surface of the skull, is shown in figs. 12 and 13, the former being an obbque front view, and the latter an obbque back view. The frontal Fig. 12. THE SKULL. 37 The frontal bone occupies the forehead and part of the temples, and extends from the eyebrows and summit of the nose to the highest point of the skull ; it is convex externally, and concave internally, and is marked 1 in fig. 12 ; its lateral limit being marked at 15, and its limit on the temple at 18. The left parietal bone is shown at fig. 12, 14.* The occipital bone, so far as it is visible on the external surface, appears at 13,1 ; this part extends across the back of the skull, being articulated with the posterior edges of the two parietal bones. At the base of the skull the occipital is bent towards the neck in the horizontal direction, and forms that part of the base of the skull resting on the summit of the vertebral column, and con- sequently containing the foramen magnum already described. The frontal and occipital bones are placed symmetrically with relation to the median plane which divides them, so that equal and similar parts lie to the right and left of it. The only part of the sphenoid bone which is superficially visible is shown at fig. 12, 18 ; a similar portion being similarly placed on the other side of the skull. The parts of this hone which are superficially visible over each ear are connected together by a continuous mass of bone which extends entirely across the skull, passing over the mouth and behind the nose, and forming, therefore, part of the base of the cerebral cavity. The visible parts of this bone, forming part of the surface of the skull, are called its wings. The temporal bones (fig. 12, 16) are similarly placed over each ear, but, as in the case of the sphenoid, the portion externally visible is only a small part of them. At its lower part, the bone turns inwards, the deflected part being inserted in, and wedged among, the other bones, which form the base of the skull. The ethmoid bone, in fine, forming no part of the external surface of the skull, is placed in the centre of its base, directly behind the nose and between the cavities of the eyes, forming the central part of the floor of the cranial cavity. 65. The Sutures. — The edges of the several bones which form the exterior shell of the skull are jointed together in a peculiar and admirable manner, so as to possess all the mechanical requisites to give security from external disturb- ance to the delicate organ which it is their especial purpose and function to protect and defend. - The edges by which the frontal, parietal, and occipital bones are connected with each other are serrated in such a manner that the projecting parts of each are inserted in the indentations of the other ; but it is evident that this, though it would prevent either from slipping under the other, by the action of a force pressing them edge to edge, would not prevent their * It will be convenient to refer in this manner to the parts of figures, as they are indicated by numbers or letters. Thus, 12, means the part of fig. 12 at which the number 14 is placed. In the same manner 40a would mean the part of fig. 40 at which the letter a is placed. 38 ANIMAL PHYSICS. separation by one which would tend to draw them asunder. Then- indentations, therefore, instead of being formed with edges converging outwards like the teeth of a saw, have the contrary form, the edges converging inwards, so that the pro- jecting pieces of the two edges are engaged in one another, like those of two boards which in cabinet-making are “ dove- tailed” together. Such a mode of connection secures the bones as well from being drawn asunder as from slipping one under the other. The solidity of their connection is further secured by another admirable arrangement. Towards the temples this dentelated and dovetailed connection is discontinued ; the edges of the parietal overlying those of the frontal towards the temples, and the edges of the temporal bones overlying those of the parietal. The contiguous surfaces of the frontal and parietal bones at the upper part, besides being dovetailed, are so arranged that the edges of the frontal overlie those of the parietal. By this arrangement every possible external disturbance is resisted ; the dovetailing resists forces which tend to move the edges of the bones towards or from each other ; while the underlying parietal edge at the summit protects the frontal from being forced inwards, and the underlying frontal and parietal towards the temple gives equal protection to the parietal and temporal respectively. The upper edges of the two parietals which pass along the crest of the skull from the frontal to the occipital are connected together in the same manner by the dovetailing of their dentelated edges. The articulations of tabular bones edge to edge in this manner are called sutures. When they are dentelated and dovetailed in the manner described above, they are called true sutures ; and when the edges overlap each other they are called squamous sutures , as resembling the scales of fishes. The occipital is connected with the parietal and temporal bones by true sutures, while the temporal and sphenoid bones, overlapping at their edges the parietal and frontal, are con- nected with them by squamous sutures. 66. Now the least reflection upon the form and disposition of the cranial bones, as already described, will render manifest the 'great mechanical advantages which result from these latter modes of connection. The wings of the sphenoid overlapping at their edges the several lateral plates of the skull, bind them together in the same manner exactly as the walls of a building THE SKULL. 39 are held together by the tie-beams of the roof, which are extended transversely between them ; for it will not be for- gotten that these great wings of the sphenoid, while they over- lap the other bones at their edges, are themselves immovably connected together by the bony arch which passes across the base of the skull, from ear' to ear. Such an arrangement may also be compared in its mechanical effect to the iron rods which are sometimes carried transversely between the walls of a building, connecting together plates or bars on the outside of the walls into which they are screwed. The temporal bones, although they are not connected by an arch extending across the skull, like the sphenoid, fulfil, nevertheless, the same mechanical functions. At their squamous suture they overlie the parietal, and, as already described, turning inwards at their lower parts, they enter into the base of the skull, among the bones of which they are firmly wedged and dovetailed ; they therefore hold together the lateral plates, which they overlie, quite as effectually as if they were connected together, like the sphenoid, by a continuous bony arch. 67. The two compact tissues which form the exterior and the interior surfaces of the bones of the skull differ in their constituents — the internal having a greater proportion of the calcareous element ; and they have consequently a corresponding difference in their mechanical properties, the exterior being tough and less hard, and the interior hard, brittle, and vibra- tory. It has been observed by Sir Charles Bell, that, as a necessary consequence of this difference of structure, there is a difference of articulation in the two systems. The inner coating, called, from its brittleness and hardness, the viU'eous table of the bone, is everywhere united edge to edge by simple apposition, neither true nor squamous sutures existing between them. Such modes of connection could only be applied to a material having toughness and a certain degree of softness. Thus, the carpenter and cabinet-maker connect their materials by dovetailing, or by tenon and mortise, which would be wholly inapplicable to the materials upon which the stone-cutter and glazier work. Such a connection made in marble or glass could not be permanent, since the brittleness of the material would soon cause the projections and dentelations upon which the security of the joint depends to be chipped off. So inevitable is this effect in such materials, that, even where no mechanical joint, properly so called, is established, precau- tions are taken against it. The edges of the cylindrical 40 ANIMAL PHYSICS. blocks of marble which form a column in architecture are prevented from coming into contact, and from splitting or chipping off in consequence, by the interposition of thin plates of lead. The purpose of the difference of structure between the exter- nal and internal tables of the skull bones is evident. The external tables, being tough and more or less yielding, are adapted to resist such forces as would tend to crack the bone ; while the internal tables, being hard, but brittle, would resist a piercing or cutting instrument, although they might be fracture-1 by a blow. 68. On viewing the interior or concave surface of the shells of bone, by the connection of which the vault of the skull is formed, expedients will be perceived for strengthening it so as to enable it to resist external disturbing forces, which bear a striking analogy to similar contrivances in architecture. Every one is familiar with the effect of groining. A groin is an expedient for strengthening a roof, by a projecting ridge formed by the intersection of two arches whose concavities are in different directions. On the interior of the skull we find one rib, or groin, extending along the middle of the head, from the frontal bone to the projecting part of the foramen magnum, and another which intersects this at right angles, passing across the occipital bone. The point of intersection of these two groins is the thickest and strongest part of the skull, and with reason, since it is most exposed, being the part which would strike the ground in falling backwards. The base of the skull is strengthened upon the same prin- ciple. “It is,” says Sir' Charles Bell, “like a cylinder groin, where the rib of an arch does not terminate upon a buttress or pilaster, but is continued round in the completion of a circle.” In fact, the skull may be considered analogous to those arches which, instead of being sustained by abutments, are supported on inverted arches built into their foundation. By such expe- dients, the base of the skull, which, so far as regards its material, compared with the roof, is thin and feeble, acquires, nevertheless, sufficient strength to resist the shocks and concussions which are incidental to its connection with the spinal column. The various bones of the head which have been described above, as well as others which support the organs and soft parts of the face, are immovably articulated together, and in this respect they form an exception to the general structure of the skeleton ; for, even after all the softer parts have been FACIAL BONES. 41 removed by decomposition or otherwise, the bones of the skull continue for an indefinite time to hold firmly together. In disinterring skeletons, when graves are opened, while all other parts are dismembered, the skull retains its integrity. Indeed the dismemberment of 'this part of the body without fracturing its component bones, requires no small exertion of anatomical and mechanical skill. 69. Facial Bones. — The bones of the face, which, with the exception of that of the lower jaw, are immovably articulated ■with each other, and with those of the skull, present five cavities, leading by as many canals and foramina to the internal chamber of the skull. In these cavities are lodged the most important of the organs, the two uppermost containing those of vision, the nasal cavities the olfactory organs, and the buccal cavity, those of taste, mastication, and articulation, as well as a part of the apparatus of voice, respiration, and deglu- tition. Since, however, these organs will severally come under review in a subsequent part of this work, we shall dismiss them for the present. 70. The lower jaw, the only movable bone in the head, shown in fig. 14, has a horse-shoe shape, the convexity being turned towards the front of the mouth. At the inner extremities towards the ears the bone is turned upwards (fig. 14,'), forming two parts called rami, or branches, at an obtuse angle with the general direction of the horse-shoe. The upper edges of these branches are formed into concave cavities, technically called the sigmoid notches, from some resemblance of their outline to one of the forms of the Greek capital letter sigma (2 or C). The posterior horn of this notch (14, 13) is a hemispherical protuberance called the condyle (fig. 14, 1J), which forms the joint upon which the jaw moves. There are various grooves, foramina, and processes indicated in the figure for the attachment of muscles and the passage of nerves and blood-vessels, which need not be more particularly noticed here. 7.1. Teeth. — At the moment of birth, twenty teeth already formed and ossified are deposited, ten in the lower and ten in the 42 ANIMAL PHYSICS. upper jaw, but are completely covered by the gums. The mouth is thus constituted exclusively for application to the mother’s breast and for the suction of milk from it, and the stomach and intestines are organised in accordance with this for the due digestion of that aliment. The constituents of the healthy milk of woman are the same as those of the body of the child, and enter into its composition in a corresponding proportion. By the process of digestion, they are distributed among the several organs of the child’s body, each passing to that for whose sus- tenance and growth it is fitted. At the age of from six to ten months, the first teeth penetrate through the gum, and towards the end of the second year the entire number have appeared. These twenty teeth are classified according to their peculiar forms, as incisors, canines, and molars. The incisors are chiselled, the canines pointed, and the molars present a broad and rough summit. Alien the mouth is closed the molars of the upper jaw, corresponding in position with those of the lower, rest upon them ; but the lower incisors and canines lie within the edges of the upper ones. In each of the jaws, there is, however, space for sixteen teeth, and consequently three places at each side remain unoccupied. The relative arrangement of this set of teeth is shown in fig. 15, where the incisors are indicated by 1 ; the canines by c, and the molars by m ; the unoccupied spaces being marked " The first teeth which break through the jaw, are the middle Fig. 15. incisors t1 i1 ; these are succeeded in regular order by the lateral incisors i5 r, the canines c c, and the molars m' m1 aud M* m*. THE TEETH. 43 Tliese temporary teeth begin to be removed at about the age of five or six years, to make way for the permanent set. The temporary molar teeth in each jaw are replaced by permanent teeth called bicuspids ; and four molars issue from the gum in each jaw, two at each side, occupying the first two of the three vacant places marked " in fig. 15, the first or anterior of these frequently appearing first of all the permanent teeth ; and at a more advanced age, two other molars fill the last vacant place in each jaw, marked "in fig. 15. Thus, a set of sixteen permanent teeth is established in each jaw (fig. 16). The last four molars, which emerge at a period Fig. 16. of life much later than the others, have been for that reason vulgarly called wisdom teeth. The periods of the successive emergence of the permanent teeth are, according to Cartwright, as follows : — Middle incisors of lower jaw (i,1), and first molars (si,1) . 5 to 7 Middle incisors of upper jaw . . . . 6 to 8 Lateral incisors (i,*) . . . . . . 7 to 9 First bicusxnds (b,1) . . . . 8 to 10 Canines (c) . . . . . 9 to 12 Second bicuspids (b,-) . . . . . 10 to 12 Second molars (m,2) . . . . 12 to 14 Third molars (m,3) (wisdom teeth) . . . 17 to 25 The teeth, which vary in their forms as well as in their functions, are represented extracted from the jaw in fig. 17, 44 ANIMAL PHYSICS. being numbered in their order, commencing from the median line backwards, and are denominated as follows : si3* SP M- B3 in c p i« Fig. 17. The function of the front teeth, as their name implies, is to cut or break the food into morsels of a magnitude suitable to the mouth ; and that of the posterior teeth to bruise and grind it, mixing it at the same time with the saliva secreted from glands under and around the tongue, by the action of which organ the pulpy food is thrown from side to side to be further ground and masticated. In the upper jaw, which is fixed, there are implanted an equal number of teeth, similarly formed, and in corresponding positions, so that in jaws properly formed and jointed each molar tooth of the lower jaw, when the mouth is closed, comes directly under the corresponding tooth of the upper jaw. The same super-position of the incisors would not be convenient ; their sharp edges not affording a mutual bearing of sufficient breadth. In jaws properly formed, therefore, the edges of the lower incisors fall within those of the upper, the internal surface of the latter resting in contact with the external surface of the former. The very exceptional cases in which the lower incisors fall outside the upper must be considered a deformity giving a peculiarly disagreeable appearance to the face. 72. Chin.— It will be observed that the external surface of the horse-shoe below the incisors is inclined outwards at a slightly obtuse angle with the teeth. This form is an exclusive characteristic of the human species, and not found in any other class of animals. The maxillary bone in all other animals is inclined more or less backwards, the teeth being thus set in the projecting edge of the jaw. The same peculiarity, though not so conspicuously apparent, * The hooked fang of this tooth is an accidental malformation of not unfreguent occurrence. JAWS AND CHIN. 45 is found in the upper jaw, which, from the teeth to the base of the nose, is vertical, while in inferior animals it is inclined backwards. Hence may be explained the peculiarly disagreeable impres- sion produced by that species of deformity presented in excep- tional cases, in which the chin instead of inclining forwards retires backwards. 73. Nothing can be more admirable than the mechanism by which the under jaw is connected with the skull. Sockets are provided in the skull, corresponding in form to the con- dyles (14, in which these play with a hinge-like motion by which the jaw is moved vertically upwards and down- wards, and the mouth opened and closed. But as the spherical form of the condyle gives to the joint, in a limited degree, the character of the ball and socket, the jaw has a certain small play laterally as well as forward and back- ward, by which the summits of the teeth of the lower can be rubbed in any direction against those of the upper jaw, so that the food between them can be treated exactly as grain is between two millstones. It is evident that for this purpose a horizontal play not exceeding the breadth of the teeth will be sufficient. The connection of the lower jaw with the skull at the head of the condyle is shown in fig. 18, where it is inserted in a V ' / most perfect freedom of motion, and is exempt from all liability to jar, arising from the conflict of the bony surfaces. But if no expedient were provided to hold tlio jaw in the position here described, it would obviously fall out of its sockets by its weight. It requires, therefore, to be retained in its position, not only vertically, but laterally, so that it may not be liable to fall by its weight, or still more by the force of mastication, but may also be secured against derangement by lateral forces acting on it by the mutual pressure of the teeth in the latter process. These objects are attained by tying the cavity corresponding with it in form, the cavity being lined with a cartilaginous cushion (18, ■*), which is therefore in- terposed between the bony sur- faces, and being lubricated by synovia, secreted by an invest- ing membrane, the jaw has the Fig. IS. 46 ANIMAL PHYSICS. jaw to the skull by three ligamentous cords, one of which holds it up, and the other two bind it to the skull on the inside and outside, leaving it, however, sufficient play for the purposes of mastication and speech. 74. One of these cords (18, 2), called the stylo-maxillary liga- ment, connects the lower angle of the horse-shoe with the part of the skull near the ear. One of the other two cords, that which binds the jaw laterally and externally (19, 1), is attached to the external edge of the branch of the jaw, and carried from thence to a point of the skull above the inner part of the upper jaw. A similar cord is similarly applied on the inside of the bone. 75. The several muscles by which the jaw is moved being attached to points upon it at different distances between the condyle and the incisors, act upon the jaw as a lever, with a different degree of mechanical advantage, according to the part of the jaw at which the resistance is applied. Thus, if the resistance be applied at any point between the insertion of the muscle and the condyle, the lever is of the second kind, and the power acts with mechanical advantage on the resistance. If the resistance be applied directly over the insertion of the muscle, the power acts with its full effect upon it as if it were applied directly to it without the intervention of a lever. These conditions are applicable to mastication with the posterior molars. But if the resistance be applied at any point anterior to the insertion of the muscle, as will be the case if it be applied at the incisors, the canine or lesser molars, the contractile force of the muscles null act upon it with mechanical disadvantage, the lever being of the third kind, and the leverage of the resistance greater than that of the power ; and this disadvan- tage will be so much the greater as the point of resistance is nearer to the first incisor. Thus we see how admirable is the adaptation of the me- chanism of the jaw to its appointed functions. The incisors, whose office is merely to cut, break, or tear the food into morsels small enough for the capacity of the mouth, act with VERTEBRAL COLUMN. 47 promptitude, and have large play but little mechanical force, — little being for such purposes needed. But when the morsels are transferred backwards to the molars to be ground to a pulp, and intimately mixed with saliva so as to be prepared for the chemical action of the stomach, they are brought to a part of the jaw where the force of the muscle is thrown upon them, first, with its full effect, and afterwards with an intensity augmented by increased leverage ; and the food is submitted to a bruising pressure and a grinding process which produce upon it the mechanical changes necessary to prepare it for healthy digestion. 76. The Vertebral Column. — If the head be a subject of great interest and importance as being the seat of the principal organs of sense, thought, and feeling, the spinal column is scarcely less so, as being at once the channel of communication of feeling and will between the head and the rest of the body, the pillar by which the head is sustained, and to the summit and base of which are appended the members of prehension and locomotion, and the shaft to which the whole framework of the trunk Is attached. Let us consider for a moment the complicated conditions which this columnar shaft is required to fulfil. First, it is a hollow tube, containing within it one of the most delicate and important organs of the body, the spinal cord, which must be protected, not only from all external disturbance, but also from all injurious strain or flexure : means of exit, meanwhile, must be given to the thirty-one pair of lateral nervous processes issuing from that cord. Secondly, planted in the pelvis, a concave mass of bone extending from hip to hip, in the same manner as the mast of a vessel is fixed in the keelson, it must have the strength and stability of a pillar to support upon its capital the head and the entire weight of the trunk and superior members attached to it from the neck to the lower extremity of the back. Thirdly, it must have flexibility and freedom of play sufficient to accommodate the necessary movements of the body and the head,' and not more flexibility nor more play than is compatible with the due preservation of the spinal cord and of the nerves which diverge from it, and the blood-vessels which enter it and issue from it. Fourthly, it must have elasticity in the vertical direction suf- ficient to intercept all incidental shocks and concussions which take place at the lower part of the body, and to prevent them from arriving injuriously at the brain. As ow, if the problem to contrive such a column were proposed 48 ANIMAL PHYSICS. to a human artificer, he would, without hesitation, pronounce that the conditions were reciprocally incompatible, and that the objects to be accomplished could only be attained imperfectly by a mutual compromise. Thus, the mechanical expedient, which would seem fitted to supply solidity to the column sufficient to support the weight of the trunk and the head, would be pronounced incompatible with any conceivable expe- dients which would give it flexibility in all directions, and still more so with any which would give it vertical elasticity. On the other hand, the due protection of an. organ so delicate as the spinal cord, with upwards of sixty nervous ramifications equally delicate, for which orifices must be provided, to say nothing of blood-vessels still more numerous entering into and issuing from every part of the sides of such a tubular column, would seem even more incompatible with the quality of flexibility and vertical elasticity than solidity and stability itself ; and, viewing such complicated conditions, it would create no surprise that the most consummate mechanician should pronounce the construction of such a column impracticable. Nevertheless, the problem has been perfectly solved, and all its apparently incompatible conditions completely fulfilled ; a circumstance which can create no surprise, however much the result may excite admiration, when it is remembered that the vertebral column is the work of One, before whose attributes difficulties cease to exist, and at whose word impracticabilities become realities. 77. This fine piece of mechanism then consists of a series of ring-shaped bones placed one over the other, like the stones of a column. But, since the annular surfaces would supply an insuf- ficient extent of mutual bearing to give the required stability, a mass of bone of semicylindrical form is attached to the inside of each of these rings. The form of this mass, which is called the body of the vertebra, may be conceived by imagining one of the cylindrical blocks which form a column to be divided by a vertical plane, passing through its axis or nearly so, the flat vertical surface produced by such a section being rendered slightly concave, so as to correspond with the form of the inner surface of the ring of which it forms a part. Tlio upper and lower semicircular surfaces of such a block, being flat, will supply a sufficient basis of mutual support. To obtain the first idea of the superstructure of the spine, we must therefore imagine a column built by placing one upon the VERTEBRAL COLUMN. 49 other a series of annular bones like that just described, each of which has attached to it a semicylindrical block of bone, the superior semicircular surface of which supports the inferior semicircular surface above it. 78. But such a column, though it might have solidity and stability, owing to the magnitude of the semicircular surfaces of the bodies of the vertebrae which rest one upon the other, and though it would present, by the exact correspondence and appo- sition of the openings of the rings, a tubular passage which would protect the spinal cord, would have no provision for the exit of the numerous nervous ramifications issuing from the one side and the other of that cord. This object would, nevertheless, be easily attained by forming on the edges of the bony rings at either side, notches in corresponding positions ; so that, when two such rings are superposed, the lateral notches in the upper edge of one coinciding with those in the lower edge of the other would, by their combination, produce a lateral orifice on each side, which would offer exits for the nerves and vessels passing from the spinal cord to the parts of the body, exterior to the vertebral column. Such lateral notches are accordingly provided in the edges of the rings, and the openings formed by their combina- tion when the lings are superposed are called the intervertebral foramina. Each of the pairs of spinal nerves, formerly described, issue accordingly through these holes, and proceed thence to them several destinations. 79. Such arrangements would fulfil the conditions of the stability of the column, the protection of the spinal cord, and would supply suitable conduits for its ramifying nerves. But it would obviously be altogether destitute of flexibility and vertical elasticity. It could only be bent by separating, more or less, the contiguous surfaces of the vertebrae, and thus exposing the spinal cord. It would be so entirely destitute of vertical elasticity, that the momentum of any blow or concussion on the lower part of the body would be propagated with unmitigated force to the head ; so that, if a person leaping came with his feet to the ground, producing a momentum proportionate to the weight of the entire body multiplied by the velocity with which it would strike the ground, the head would suffer the concussion which would be produced by a fall on the head, instead of on the feet. The expedients by which such an evil is prevented are as admirable as the other provisions in the mechanical structure of the body. The contiguous suifaces of the vertebras are not E 50 ANIMAL PHYSICS. in immediate contact. Between the fiat semicircular surfaces of their bodies, thick discs of cartilaginous substance are in- terposed, which are firmly adherent to the semicircular sur- faces of the bones. These discs are highly elastic and resisting, and are also more or less flexible ; so that the surfaces which they connect are not only capable of moving through a certain limited space towards and from each other, maintaining their parallelism, but are also capable of being inclined to each other on any side, by the compression of the interposed disc of car- tilage on one side and its extension on the other. Thus, by the same expedient, the column receives at once flexibility and vertical elasticity, without being in the least deprived of so- lidity and stability, inasmuch as the firm adherence of the interposed cartilaginous discs to the surfaces of the vertebrae effectually prevents all lateral displacement. The same expedient secures the fulfilment of the other condition of the spinal problem ; since, by stopping up the in- terstices between the parallel vertebral surfaces, the spinal cord is as completely enclosed as if it were protected by a con- tinuous tube of bone. When it is considered that this column has to support the entire weight of the trunk, the head, and superior members, it will be easily conceived that, however tenacious these interposed cartilages may be, they cannot be regarded as altogether suf- ficient to resist the strains in all directions which so great a weight, liable in virtue of the flexibility of the supporting column constantly to shift its direction, would produce. Nu- merous other accessory expedients are accordingly provided, adapted by their structure and position to resist these various strains. 80. Along the front or convex side of the column, each ver- tebra is strapped to those which are above and below it by strong fibrous ligaments, wliich are inserted in the middle of the con- vex side of the body, and which are accordingly stretched tightly over the edges of the intervertebral fibro-cartilaginous discs. To afford further strength and security, another series of similar ligamentous straps are extended over these, connecting in pairs every third vertebrae ; and, to render assurance doubly sure, another series of similar straps stretched over these last connect the body of each vertebra with that of the fourth or fifth above or below it. This series of fibrous straps, extending longitudinally over the entire length of the front of the column, is called by VERTEBRAL COLUMN. 51 anatomists tlie anterior common ligament, and its mechanical purpose is evidently to resist all backward strain of the column. An expedient similar precisely to this is applied to the pos- terior surfaces of the bodies of the vertebras, which form part of the interior surface of the vertebral canal. Each vertebra is strapped in the same manner to that which is above and below it, on that side of the interior of the tube which is next the semicircular body, and it is strapped in like manner to the next but one above and below it, and so on, as in the case of the anterior common ligament just described. This system of ligamentous straps, extending longitudinally throughout the whole length of the vertebral canal along the posterior side of the bodies of the vertebrae, is called by anatomists the posterior common ligament, and its mechanical function obviously is to resist all undue forward strain of the spine. 81. The movements of the spine, like those of all other parts of the body, are produced by the contractile force of muscles inserted at suitable points upon its bony sur- faces ; but when the form of the vertebral rings and bodies described above is considered, it will be obvious that no points of insertion could be found in them which would give to the muscular power any leverage at all proportionate to the great force which must be exerted in moving the mass of matter composing the trunk, the head, and superior members, the col- lective weight or inertia of which must necessarily react against such muscles. The structure of the spine, therefore, so far as we have described it hitherto, though fulfilling so many and apparently difficult conditions, would still be faulty in relation to the action of the muscles upon the mobility of the trunk and its appendages. We find accordingly, besides the provisions already described, others, which are specially appropriated to the solution of the muscular element of the problem. From the part of the bony ring which is behind the points where it joins the body of the vertebra three levers project, two laterally and one from the centre of the posterior part of the ring. The two lateral projecting levers are denominated, from their direction, the transverse processes, and the centre lever is called the spinous process, this last being generally in- clined more or less downwards. The ends of these levers aro the points of insertion of various spinal muscles, which act 52 ANIMAL PHYSICS. upon the vertebrae in the same manner as a mechanical power acts upon a rectangular lever. By these muscles, the spine, and with it the entire trunk and its appendages, is moved in various directions, as well laterally and obliquely as in the median plane. A similar system of muscles are inserted in the anterior sur- face of the vertebral column, which may be regarded as an- tagonistic to those just described : the muscles on the posterior side raising the body and bending it backwards, while those on the anterior side cause it to incline forwards, and the muscles connected with the transverse processes incline it sidewards. The slightest consideration of the form of the body will render it manifest that its centre of gravity is in front of the vertebral column ; and, consequently, that by gravity alone there would be a continual tendency of the body to incline forwards. Such a tendency woidd obviously come in aid of the power of the muscles on the anterior side of the column, while it would oppose those on the posterior side. Hence arises the necessity for the powerful levers supplied to the latter by the spinous and transverse processes, and the absence of such aids to those in front of the column. 82. Haring thus explained the various mechanical provisions from which the spinal column derives its functions, these ex- planations will be rendered more clear and satisfactory by Transverse process Lateral notcli to") form lateral fora- J-l men. ) Vertebral Canal Flat end of the body 2 Articular process. 3 Lamina or plate. Spinous process. 4 Lamina or plate. 5 Articular process. Transverse process. 6 Lateral notch to form lateral foramen. Fig. 20. figures illustrating the several parts of that column and its appendages. In figure 20, a view of a vertebra is shown, the line of sight being supposed to be at right angles to the fiat semi- circular surface of the body : and the names of the several VERTEBRAL COLUMN. 53 parts are indicated so plainly in tlie figure, that, after what has been explained, they cannot fail to be understood. The spinous process as here shown must be understood to be inclined more or less downwards from the general plane of the figure, and therefore to be foreshortened. The two promi- nences with flat surfaces, called articulai processes, are those by which each ring is connected with that which is above or below it. They are not generally in the direction of the semicircular surface of the body, but more or less oblique, and in some cases nearly at right angles to it. But, whatever be their direction, those of two vertebrfe superposed correspond in position, figure, and magnitude, so that they come face to face parallel to each other. They are coated with cartilage, like other articulations, and are surrounded with a synovial membrane, by which they are lubricated. The pieces of bone connecting the body with the transverse processes are called pedicles ; and it is in the bor- ders of these, above and below, that the notches are formed by the combination of which the lateral foramina for the passage of the spinal nerves are formed. A side view or profile of one of the vertebrae, taken from the middle of the back is shown in fig. 21, where 3 is the body ; 1 and 2 the lateral notches ; 4, 4, arti- cular surfaces to receive the heads of the ribs ; 5, 6, the ar- ticular processes described above, which in this case are very oblique ; 7, the transverse process ; 8, an articular cavity for the insertion of the tubercle of a rib ; 9, the spinous process, inclined downwards ; for the insertion of the muscles. 83. A general view of the spinal column, divested, however, of its ligaments, intervertebral cartilages, and all appendages not strictly osseous, is shown in fig. 22. Fig. 21. 10, the tuberculated end 84. The component vertebra are designated according to their numerical orrier, counting downwards from the highest, which immediately supports the head. They are usually grouped in three classes, and named according to their neighbouring parts. Thus, the first seven are called cervical, the next twelve dorsal, and the last five lumbar, making in all twenty-four distinct vertebra in the adult. In infancy there are nine others, the first five of which, at mature age, are connected together by the ossification of the intervening cartilage, and the bone formed by the combination is called the sacrum. The four last, in like manner, coalesce by ossification and 54 ANIMAL PHYSICS. form a single bone, called the coccyx, from a fancied resemblance to the beak of a cuckoo, for which bird coccyx is the Latin name. o P3 Transverse process. Spinous Process- Spinous process. Transverse process. Cervical vertebne. 12 Dorsal vertebra. O z j ;} Lumbar vertebra. ) Sacrum. Coccyx. Fig. 22. 85. Thus it appears that the spinal column in mature age consists of twenty-six bones, of which twenty-four are vertebra, and two others have the forms shown in fig. 22, being called the sacrum and coccyx. In its VERTEBRAL COLUMN. 55 normal position, the body standing upright, the vertebral column has cur- vatures, as shown in the figure, somewhat resembling an italic f. It is convex towards the front, in the cervical and lumbar, and concave in the dorsal and sacral regions. The changes of curvature are made by what in geometry is called a point of inflexion, the convexity passing gradually and insensibly into concavity and vice verad. At the point, however, where the lumbar region terminates, the change of curvature is more sudden, the commencement of the concavity of the sacrum making very nearly a right angle with the terminal direction of the lumbar region, so that the sacrum is very nearly horizontal. This circumstance is of extreme importance in the mechanism of the body ; for it is evident that stability could not be secured if the whole weight of the body were to rest vertically on a point such as that of the coccyx. The sacrum, however, being bent backwards in a direction slightly oblique to the horizontal line, and having considerable breadth in its transverse dimen- sions, forms a sufficiently extensive base for the column. 86. A front view of the sacrum is shown in fig. 23, where 1, 1 are the ridges which originally separated the component vertebra?, but which in adults are ossified ; 2, 2 are the foramina for the exit of the nerves ; 6, the articular surface by which the sacrum is attached to the lowest lum- bar vertebra ; and 8, the point of connection of the sacrum with the coccyx. 87. The vertebral column is planted firmly between two large and irre- gular-shaped bones called the ossa in- nominata, or nameless bones, from their want of resemblance to any fa- miliar object. These bones surround the lower part of the body at the hips, forming the annular cavity called the pelvis. The sacrum is wedged between them at the back, and the bony circle is completed in front by a bone called the pubis. The connec- tion of the sacrum with the pelvis is shown in fig. 24 by a vertical section made through the centre of the sacrum and lumbar vertebra?, showing the left os innominatum. Sir Charles Bell compares this implantation of the spine in the moveable base of the body to the insertion of the mast in the keelson of a vessel, the sacrum being the step on which the base of the pillar, like the heel of the mast, is socketed and mortised. It is further secured by being tied down to the lateral parts of the pelvis by powerful ligaments, which may be compared to the shrouds by which the mast is bound to the sides of the vessel. These ligaments secure the lower Fig. 23. 56 ANIMAL PHYSICS. part of the spine against the effects of lateral motion or rolling. Fig. 24. 88. It has been already explained generally that the spaces between the vertebrae are filled by elastic cartilaginous discs ; the form and mechanical effects of these will be more clearly understood by reference to the following figures. In fig. 25 is represented the surface of one of these cartilaginous discs, placed upon the body of a ver- tebra. It consists of a series of concentric rings of cartilage, 25, ', placed one within the other, the interstices between them, as well as the central space, fig. 25, :, being filled with highly elastic pulpy matter. In fig. 26, a section of this disc made through the median plane is represented, the disc being sup- posed to be as in its natural state, compressed between two vertebras. It will be seen that the concen- tric cartilaginous rings, 26, ', which are near the external part Fig. 25. of the vertebra, are bent with their convexities outwards, while the internal ones, 26, •, have their con- VERTEBRAL COLUMN. 57 verities inwards. The pulpy matter, 26, 3, is so elastic that, when it is relieved from the incumbent pressure of the upper vertebra, it rises up so as to assume a conical form. Fig. 26. The fibres of each of the intervertebral cartilaginous rings are found to be extended obliquely between the vertebra, being firmly attached at their extremities to the surfaces with which they are in contact. The direction of their obliquity varies from layer to layer, in one running from right to left, and in the next the reverse, fig. 27, ', e. Fig. 27. What has been hero explained will render evident the admi- rable manner in which the spinal column acquires flexibility and elasticity, without impairing its stability or diminishing the protection afforded by its tubular canal to the spinal cord. The flexibility required is not the same in all parts of the column, more pliancy being obviously necessary in the cervical and lumbar than in the dorsal region. The inter- 58 ANIMAL PHYSICS. vertebral discs are accordingly thicker where most pliability is needed. Although the pliability existing between any two contiguous vertebraj be inconsiderable, yet the combined effect of all gives to the column all the flexibility which is required. With regard to the vertical elasticity by which the force of concussion, taking place in the lower part of the body and propagated upwards, is mitigated, it is important to observe that the elasticity of the intervertebral discs are not the only, nor even the most effectual provision against this injurious effect. The curvature of the spinal column, combined with its flexibility, is a much more effectual means of intercepting such shocks. If a highly elastic steel spring were pflaced vertically, the elasticity would either not yield at all to a pressure on its summit, or would give way with a sudden jerk, the spring passing instantaneously from the straight to the curved form. But, if such a spring, instead of being straight had the form of an italic /, it would yield gently and gradually to any force suddenly exerted upon either of its ends. The spinal column derives from its curved shape the same virtue. A sudden con- cussion acting upwards upon its base, instead of being trans- mitted without mitigation to the brain, is intercepted partly by the momentary compression it produces in the interver- tebral discs, but much more by the momentary increase of curvature which it gives to the vertebral column. 89. The anterior common ligament, or rather system of ligaments, by which the vertebrae are strapped to- gether in front of the column, Fig. 28. is shown in the case of three contiguous vertebrae in fig. 28. After what has been explained, it will be understood that the exterior ligaments only are here visible, which connect distant vertebrae ; those which connect the nearer or conti- guous vertebrae being under them. The ligaments by which the heads of the ribs are bound to the vertebrae, are shown in fig. 28, 2. The posterior common ligament, such as it has already been described passing down the interior of the vertebral canal, is VERTEBRAL COLUMN. 59 shown in fig. 29 3 ; the ring forming the remainder of the canal being sawed off to render it visible. There are various other ligaments by which the rings around the vertebral canal are tied together, the principal of which, called the supraspinous lic/ament, connects the extremities of the spinous processes one with another, forming a continuous cord, or rather series of cords, extending from the lowest cervical vertebra to the base of the column. Like the anterior and posterior common ligaments, this consists of a system of superposed ligaments, the innermost con- necting contiguous processes, the next alter- nate processes, and so on. 90. When the great freedom of motion possessed by the head in all directions, as well by inclination as by rotation, is considered, it may naturally be expected that the expedients by which it is connected with the vertebral column must present an ex- ample of a curious and interesting piece of mechanism ; since it must not only provide for the free play of the head, but also for the security of the innumerable nerves and blood- vessels, and the conduits of the organs of voice, respiration, and nutrition, with the trunk. The mechanism of the articulation of the head with the trunk not only includes the consideration of the form of the bony border of the foramen magnum, already described in the base of the skull, but also those of the first two vertebrae, which, as we shall now see, differ essentially from the other vertebrae in points which have immediate reference to the cervical articulation of the head. 91. The first vertebra, upon which the head immediately reposes, and called from that circumstance the Atlas, is shown by its upper surface in fig. 30, 1 being its ante- rior, and 4 its posterior side. It differs obviously from the other vertebra; in having no semi- cylindrical body in front, the space which such a body fills in the infe- rior vertebra; being cut open so as to enlarge towards the front the aperture enclosed by the vertebral ring. Fig. 29. CO ANIMAL PHYSICS. The spinous process is also wanting, or may be regarded as being repre- sented in a rudimentary state by the slight projection, 30/. The flat surfaces, 30/, are those upon which the articular surfaces of the skull bor- dering the foramen magnum rest, and with which they are articulated. The opening in the atlas being much wider from front to back than the diameter of the spinal cord, the latter passing through the posterior part leaves a large portion of the aperture unoccupied in front, extending from 30, • to 30, 3. 92. To explain how this space is filled, it will be necessary to consider the form of the second vertebra, called the ox is, a side view of which is presented in fig. 31. This, like the atlas, diflfers essentially from the other vertebrae. The body in front, instead of being cut off to a flat surface flush with the general level of the bone, is continued upwards, 31/, somewhat in the form of a tooth, from which it has been called the odontoid process. When the axis is placed under the atlas, so that its foramen corresponds with the posterior part of that of the atlas through which the spinal cord passes, this odontoid process, 31/, passes into the front part, 30, 3, of the aperture of the atlas, leaving a small space between it and the spinal cord. Across this space, binding the odontoid process, passes a ligament, 32/, which divides the entire aperture of the atlas into two unequal parts, the posterior part, 32/, being appropriated to the spinal cord, and the anterior part to the odontoid process. Another ligament issuing from the former upwards, 32,*, and downwards, 32, 2, and therefore at right angles to it, is inserted Into the odontoid process below the axis, and is continued upwards to the border of the foramen magnum in the skull. This crucial liga- ment, therefore, attains at once several important mechanical purposes. By its horizontal part, it binds the odontoid process in its place, and prevents it from pressing backwards on the spinal cord. By its downward branch it binds the odontoid process, and therefore the axis, to the anterior part of the atlas, and prevents the odontoid process from slipping downwards. By its upward branch it binds both the axis and atlas to the skull. 93. The base of the skull, showing the foramen magnum and its borders, is represented in fig. 33, the lower jaw and its appendages being removed. One of the surfaces on the border of the foramen magnum, which articulate with the atlas, is 33, 15, tho correspondiug surface on the other side being indicated but not numbered. THORAX. 61 04. The atlas, the axis, and the skull are bound together by Fig. 33. other ligaments surrounding the vertebrae, which need not be further particularised here. It appears, therefore, that the odontoid process of the second vertebra is a pivot on which the first vertebra supporting the head turns ; and it is to this form of attachment that the head owes all that freedom of rotation, by which the face can be at will presented in any desired direction within the limits of the direction of the shoulders. The faculty which the head possesses of being inclined in any direction forwards, backwards, laterally, or obliquely, is due to the flexibility of the cervical vertebne. 95. Thorax. — The bony cage already described in general terms, called the thorax, is shown by a front view in fig. 34. The ribs are articulated at their posterior extremities with the twelve dorsal vertebra-, and tied to them by ligaments as shown in fig. 28. In front, they terminate in cartilaginous straps by which the first seven, counting downwards, are connected with the sternum, 34, or breast bone ; these seven are called true ribs. The other five likewise terminate in front in cartilaginous cords ; but these do not extend across the chest, each being connected with the cartilage of the rib above it, with the exception of the last, or twelfth rib, 34, l(i, which, being loose and unattached to the others, is designated the floating rib. These five last ribs, not being directly connected with the sternum, are called false ribs. 62 ANIMAL PHYSICS. This stracture of the thorax produced by the combination of the slightly movable articulations of the ribs with the vertebrae, called by anatomists amphiarthrotis, aud the flexible cartila- ginous extremities on the side of the sternum, is admirably adapted to the functions of the chest. While the great vital organs of circulation and respiration included within the thorax have adequate protection, the structure here described accom- modates itself perfectly to the never ceasing mechanical action resembling the opening and closing of the boards of a bellows, by which respiration is maintained. In their normal position, the oval rings formed by the ribs are inclined downwards from the vertebrae ; but when, for the purpose of inflating the lungs, the capacity of the chest must be enlarged, the ribs generally, and more especially the false ribs, must be drawn upwards, by which the oval rings are rendered more nearly horizontal Fig. 34. When, on the contrary, the ah- which has inflated the lungs is expired, the same bony rings are pressed downwards to their extreme limit, so as to diminish the capacity of the chest. In this manner, with every inspiration and expiration in the act of breathing, the ribs are moved alternately upwards and downwards, with an oscillating motion upon their articulations with the vertebrae ; aud the perfection of the mechanism by SCAPULA AND CLAVICLE. 63 which this is accomplished, and that by which it has a self- restoring power, will be understood, when it is considered that that mechanism retains its functions usually for above seventy years sufficiently unimpaired for the maintenance of vitality, and sometimes even for a century ; the action being maintained ■without a moment’s intermission, and being repeated thirty or forty times per minute, night and day, and sleeping or waking. 96. The Scapula and Clavicle. — At the summit of the trunk here described, a bony structure is attached, adapted for the support of the arms. This, on each side, consists of two bones, one posterior, called by anatomists the scapula, and popularly the “shoulder-blade,” and the other anterior, the clavicle, popularly called the “collar-bone.” The scapula is a large, flat, triangular-shaped bone attached to the shoulder, and occupying the upper and external comer of the back. A view of the outside surface of the scapula of the right shoulder is shown in fig. 35, the various parts of which, indicated by the num- bers upon it, have received distinct names in descriptive anatomy, which are unimportant here, and may be ascertained by reference to “ Quain’s Anatomy.” The clavicle is a slender and cylin- drical bone articulated with the sca- pula at its external and superior comer 35,6, and extending from that point to the summit of the sternum, with which it is also arti- culated. Its chief purpose is to keep at a fixed distance, asunder, the scapula and the sternum, and it has accordingly been compared, not inaptly, to the flying buttress in Gothic architecture. The two clavicles will be seen in situ by refer- ence to fig. 11, where also the scapuke are partially visible through the interstices of tho ribs. The mechanical effect of the two collar-bones thus inter- posed between the summits of tho shoulders and the breast- bone is to keep the shoulders apart and give them expansion, throwing them sufficiently beyond the vertical line of the ribs to leave free space for the play of tho arms, as will be seen by x-eference to fig. 11. 64 ANIMAL PHYSICS. Tlie clavicles, as shown hi fig. 11, are remarkably thin bone-, incapable of resisting much strain applied to them laterally. This, which to bones otherwise placed might be a defect, is quite consistent with the functions assigned to the collar-bones, which are subject to no other strain than the longitudinal thrust which may arise from the tendency of the shoulders inwards towards the breast, which they are perfectly capable of resisting. fi'T. The Arm. — At the external comer of each scapula is found a shallow, spherical cavity, called the glenoid cavil g, corresponding in form with the head of the upper bone of the arm. The edge only of this cavity, 35 14, is visible in the figure, the cavity itself being situated on the other side of the bone. The head of the upper bone of the arm articulated with it is retained there by pro- per ligaments, having, nevertheless, full freedom of motion. The upper member, popularly called the arm, consists of three parts : the arm, properly so called, or humerus, extending from the shoulder to the elbow ; the fore-arm, consisting of two bones of nearly equal length, placed in juxtaposition, called the ulna and radius : and in fine, the hand, consisting of three principal parts — the carpus, or wrist ; the metacarpus, being that part of the hand be- tween the wrist and the knuckles ; and the fingers, the bones of which are called phalanges. The humerus, shown in fig. 36, is an irregular cylin- drical bone, the superior extremity of which presents a hemispherical head, 36, 10, turned obliquely in- wards towards the shoulder, and which, when in its place, is fitted in the corresponding cavity, 35, 14 of the scapula, above described. It is shown in situ in fig. 11. The joint thus connecting the humerus with the scapula has in the highest degree the character of the ball and socket. The humerus is, by this joint, capable of taking any position within the limits of an extensive cone having its vertex in the joint. Thus, while, on the oue hand, it mai be brought into close contact with the ribs, it may, on the other, be elevated so as nearly or altogether to touch the side of the head. It may Ire presented directly forwards, or directly backwards, so that its play may more pro- perly be said to be limited by a hemisphere than by a cone. This play, however, will greatly vary in different individuals, depending on peculiar Fig. 36. ARM. 65 natural conformation, exercise, and age. In all, however, without excep- tion, this bone has a greater play than any other member of the body. This great range of motion is given to the arm partly by the form of the articulating surfaces of the shoulder-joint, and partly by the form and structure of the ligament surrounding it. While the head of the humerus, 36, *°, is hemispherical, the cavity behind, 35, 14, is a very shallow spherical depression, in which, consequently, the head of the bone can turn freely through a great range. The ligaments by which the several bony pieces are connected together at the shoulder are shown in fig. 37, which represents the interior surface of the scapula of the right shoulder, or that surface which is laid against the trunk. The collar-bone is tied to its upper and external corner by two ligaments, one of which, 37, ', appears in the figure, the other being behind the joint. This hone is also tied to the scapula at 37, 4, by the ligament 37,2. In a projecting corner of the scapula immediately under the point where the clavicle is articulated, is the shallow spherical cavity already described, in which the hemispherical head of the humerus plays, and this is loosely held in its place by ligaments, fig. 37, 5 and 8, which extend round the head of the hone. The ligamentous connection thus formed between the arm and the shoulder is so loose, that when the surrounding muscles are removed, the head of the bone would fall out of the socket. It is, therefore, by the muscles rather than the ligaments that the bone is retained in the socket, the ligaments serving merely as a check to limit its play. The atmo- spheric pressure is also a very effica- cious means of retaining the head of the humerus in its place (15). The lower end of the hume- rus, 3G, 14 and ,s, is formed into two semicircular pieces, called con- dyles, with fiat and parallel sides, somewhat resembling those of the ends at which the legs of a compass are united. This extremity is articulated with the superior extremities, or heads of the radius and ulna, whicli have a form corresponding with that just de- scribed, so that the projecting parts of one entering the cavities of the other, and being tied together by proper ligaments, an arti- culation is formed which has the mechanical properties of a hinge or cradle-joint. Thus the fore-arm can be bent to the shoulder, or straightened, so as to form a line with the humerus ; but it is prevented from being inclined backwards by a projection from the end of the ulna, called the olecranon, which forms part of the extremity of the elbow. Y 66 ANIMAL PHYSICS. By considering tliis hinge motion of the fore-arm, combined with the extensive range of motion of the humerus, it will be perceived how vast a play is given to the hand. In fact, it may be directed thus to any point placed within a large segment of a sphere whose centre is at the shoulder-joint, and who-e radius is the length of the arm. When it is considered that in man the exclusive use of the hand is for the purposes of pre- hension and touch, the importance of this provision will be 98. The Fore-Arm. — The radius, placed in juxtaposition with the ulna, is connected with it throughout its whole length by a strong fibrous membrane, and by ligaments at either end. At the lower end of the radius is an enlargement, to receive the articulation of the wrist, to which the hand is appended. The connection of the radius with the ulna is such, that it is capable of revolving round it, carry- ing with it the hand. The ulna and radius in their natural juxta- position are shown in fig. 38, which is a front view of the bones of the right fore-arm. The extremities of the radius are, 38, 7 and 9. Those of the ulna, 38, 14 and 16. The olecranon is shown at 38, 13 ; and the large notch, called the sigmoid notch (38, 15), together with the con- cave head of the radius, form the hinge-joint with the humerus. These bones may be seen in situ in fig. 11. 99. The Wrist consists of eight small bones ranged in two rows one above the other, and so placed as to surround and protect the blood-vessels and nerves which pass from the arm to the hand. For tins purpose they form, combined with the surrounding ligaments, a short and strong tube which will bear great external force without being compressed. Although each of these numerous bones has but a very limited mobility in relation to the adjacent ones, tbeircombinationgives to the hand that freedom upon the wrist which is rendered manifest in the countless examples presented of nice and delicate manipulation. The metacarpus is composed of a range of long thin bones articulated with the wrist-bones, four of which are placed parallel apparent. Fig. 3S. HAND. G7 ancl in juxtaposition, and are connected by ligaments at the knuckles, having very little independent motion, and forming the bones between the palm and back of the hand. To these bones are articulated the fingers. The first bone of the meta- carpus has more play, and is inclined more towards the palm of the hand. At its extremity the thumb is articulated in such a position that it can at will be brought in opposition to each of the fingers, a faculty which is of the highest importance in all processes of manipulation. Tlie forms and arrangement of the wrist-bones, and their positions rela- tively to those of the hand and arm, are shown in outline in fig. 39, where 9 is the radius, and 1 0 the ulna ; 1, 2, 3, 4, the upper row, and 5, 6, 7, 8, the lower row of wrist- bones ; the five metacarpal bones which extend from the wrist to the knuckles, appearing below, but cut off short. The fingers consist of three, and the thumb of two phalanges, which diminish gra- dually in length towards the extre- mities. The ligament by which the hu- merus and fore-arm are connected at the elbow, the fore-arm and the hand at the wrist, and those by which the hands and fingers are con- nected, are shown in figs. 40 and 41. Fig. 39. Fig. 40 represents a front view of the left arm and hand ; the inside of the elbow-joint, and the palm of the hand, being supposed to be presented to the observer. Fig. 41 repre- sents a back view of the same, the back of the elbow and the back of the hand being presented to the observer. It will be remembered that the radius is the outside, and the ulna the inside bone of the fore-arm, con- sequently the radius is on the rightof fig. 40, and on the left of fig. 41. The bones of the humerus and fore-arm are connected together by a ligament attached to them above and below the elbow, and surrounding the joint like a glove. Although this ligament is thus continuous around the arm, it has been found convenient in the nomenclature of anatomy, to distinguish it as four separate ligaments, — one at the front, and another at the back of the joint, and the two others at the sides, called accordingly, the anterior, 40, 3, the posterior, 41, 4, the lateral internal, 40, ', and lateral external ligaments, 41, Besides these which directly tie together the bones of the humerus and fore-arm, the radius is tied to the ulna by a remarkable ligament called the annular or orbicular, which cannot be distinctly seen in figs. 40 and 41, but is shown separately at 42, 5. The radius and ulna are also tied together partly by a membrane, 40, 6, the fibres of which extend obliquely between them, called the f 2 68 ANIMAL PHYSICS. interosseous membrane, and partly by an oblique ligament, 40/, near tbeir upper extremity. Fig. 40. Fig. 41. The lower extremity of tlie fore-arm is tied to the wrist by a con- tinuous ligament which surrounds it like a glove, being, as in the case of the elbow-joint, distinguished in anatomy as four ligaments ; theater, 40, 1U, the posterior, 41, u, the lateral interval, 40, 8, and the lateral CXtCl'TlCll 40 ^ ' The two rows of bones which compose the wrist are also severally con- nected by ligaments, which being within those just described, are not HAND. G9 apparent in the figures, but are indicated in fig. 39, where the places of the synovial membranes for the lubrication of the numerous joints are also shown. Thus, A is the synovial for the radius and ulna ; b for the radius and wrist ; c is opposite to a ligament which connects the radius and ulna, and lies between the two synovials just mentioned ; d is the synovial between 3 and 4, and e, e that between the two rows of wrist-bones, and also between the second row and the hand f being the synovial between the first bone 5, of the second row and the thumb. The wrist is connected with the hand by two ligaments, 40, 12, on the side of the palm, and 41, 13, on that of the back of the hand ; the former being called the palmar, and the latter the dorsal carpo -metacarpal ligaments. The me- tacarpal bones forming the skeleton of the hand are connectedtogether by intermediate ligaments, as well at the wrist as at the knuckles, shown at 40, 14 and 15 ; and the thumb has similar liga- Fio- 42. ments, the upper of which is shown at 40, 16. The several finger-bones are also connected by ligament, all of which are shown in figs. 40 and 41. 100. It may be remarked that the several levers, by the combination of which the superior members are formed, diminish progressively in length in proceeding from the trank to the extremities. Thus, the humerus is longer than the fore-arm, the fore-arm than the metacarpus, the metacarpus than the phalanges, and each phalange than that which succeeds it. The advantage of such an arrangement is easily perceived ; the articulations increasing in number in proportion as we approach the extremity of the member, give the greatest facility in directing the instrument of prehension to the object it is intended to seize. Thus, the humerus makes, as it were, the first rough .approach to the object. The fore-arm, by its inflexion on the humerus, comes nearer to it ; the hand revolving on the ulna by means of the radius, the phalanges, with motions succes- sively smaller and nicer, are brought nearer and nearer to the object until it is seized. Every one who is familiar with the mechanical expedients adopted in philosophical instruments, will perceive the striking analogy between these arrangements and the mechanical contri- vances by which the most extreme precision is attained in the observations made with them. There are two classes of adjust- ments always provided, designated by instrument-makers and observers as the coarse and the fine adjustments. By the coarse 70 ANIMAL PHYSICS. adjustment, the instrument is brought promptly, but only approximately, to the desired position, which is afterwards attained with precision by the fine adjustment. Thus, in the mechanism of the arms, the motion of the humerus and fore- arm are the coarse, and the smaller and more delicate motion-, of the fingers, the fine adjustments. 101. The I*eg. — Between the lower members and the upper there are obvious analogies ; thus, the hip and the shoulder, the knee and the elbow, and the ankle and the wrist, severally p> re- sent resemblances of structure which are strikingly apparent. But, on the other hand, there are differences between the arms and legs, and their mode of articulation, which are not less striking, all of which have relation to the peculiar functions of each member, the superior being adapted exclusively for prehen- sion, and the inferior as exclusively for locomotion. The pelvis, as already explained, is a shallow basin, concave upwards, the bottom of wliich is open, or rather filled with muscular tissue ; it forms the base, as the scapulse and clavicles form the summit, of the trunk. The two pelvic bones, already mentioned, called the ossa innominata, placed partly behind and partly at the sides, forming the hips, are analogous to the scapuhe which form the shoulders, and they are connected in front by the bony arch called the pubis, exactly as the scapulae are connected by the clavicles. This will be perceived more evidently by referring to fig. 11, where all the bones are shown in their proper relative position, with their names annexed. 102. At the external corners of the hip-bones are two deep hemispherical cavities looking obliquely downwards, intended to receive the spherical head of the femur, or thigh- bone. This bone, which is the longest and the largest in the skeleton, extending from the hip to the knee, is represented in fig. 43, and consists of a rather irregularly cylindrical shaft, 43,1, terminating above by a hemispherical head, 43/, formed upon a neck, 43, 4, bent obliquely inwards towards the hip, and below by an enlargement, 43,"’ and 12, at the bottom of which there is a cavity, 43, s, of which the vertical section made by a plane passing from front to back is circular, being the form required to produce a liinge-joint. The direction of the neck, 43, 4, is such that the head, 43, 5, is presented to the spherical cavity already described at the corner of the hip ; and the joint thus formed has, therefore, the cha- LEG. 71 racter of the ball and socket, and is analogous to that of the shoulder. Between them, however, there is a difference of form, which has an evident relation to their peculiar functions. The socket of the shoulder,; as has been already explained, is extremely shallow, and consequently gives a veiy wide play to the arm. But this shallowness gives little security to the maintenance of the bones in then- relative position, which is therefore effected altogether by the liga- ments and surrounding muscles ; greater strength of connection not being required, since the bone and socket are not urged together by any considerable pressure. In the case of the hip-joint, however, the mechanical conditions are totally dif- ferent. Although, for the purposes of loco- motion, the play given by the ball and socket principle to the leg is requisite, the same large extent of play as is necessary for the purpose of prehension in the am is not at all required. On the other hand, the looseness of mechanical connection, which must necessarily attend a socket so shallow as that of the shoulder, would be totally incompatible with the firmness and stability necessary to render the legs a secure sup- port for the incumbent weight of the body. Indeed, by merely inspecting the relative position of the bones, shown in fig. 11, it will be apparent that if the socket of the hip, in which the head of the thigh-bone is articulated, were as shallow as that of the shoulder, the weight of the body alone, not to mention the thousand external disturbances to which it is exposed, would have a continual tendency to produce its dislocation. The hip-joint is surrounded by a ligament and other fibrous coverings, which connect together the bones in the same manner as the ligaments already described connect together those of the shoulder. But in the case of the hip there is an additional liga- ment, called the cotyloid, which is a fibrous ring surrounding the edge of the spherical socket, increasing its depth, and com- pleting its border where the bone is deficient. i ! Fig. 43. 72 ANIMAL PHYSICS. 103. The leg, which extends from the knee to the ankle, con- sists like the fore-arm of two bones, placed parallel to each other, in contact at their ex- tremities, but separated by an intermediate space throughout their length. One is much thicker than the other, and is called the tibia, from a Latin word, which signifies a pipe or flute. The other is comparatively thin, and is called the fibula, a Latin name, signifying the pin of a brooch, and sometimes the peronea — a Greek word, having the same signification. The resemblance from which this name has been taken will be apparent by reference to fig. 44, which presents a front view of the right leg, the tibia, 44,13, and the fibula, 44, 14. The head of the tibia, 44, 4, terminates at the superior part by two surfaces, 44, 4 and 6, coated with cartilage, between which is an eminence, 44, ', which articulates with the lower extremity of the thigh- bone. When the tibia is articulated with the femur, the projecting parts of the latter, 43, 9 and ", rest upon the cartilaginous coatings, 44, 6 and 5, the eminence, 44, ', entering the cavity, 43, s. This articulation is covered in front by a bone called the ‘patella, or knee-pan, on which the tendons of the muscles which move the leg play like ropes over a Flff- 44. fixed pulley. At their lower extremities the tibia and fibula throw out two processes or prominences, 4 4, 8 and L'°, which form the ankles, the former being the inner, and the latter the outer one. Unlike the bones of the fore-arm, those of the leg do not admit of revolution one round the other ; and the foot, instead of being articulated with the fibula, which corresponds to the radius, is more immediately attached to the larger extremity of the tibia. 104. The Foot. — Tlie foot, like the hand, consists of three principal parts ; the tarsus, or upper instep, extending from the ankle to the point where the foot turns horizontally ; the metatarsus, or lower instep, extending from the latter point to the origin of the toes ; and, in fine, the toes, the bones of which, like those of the fingers, are called phalanges. A view of the bones of the foot seen from above, is shown in fig. 45, and one as seen from below in fig. 46. The highest bone, 45, u, of the instep, resting upon all the others, is pulley-shaped, its vertical section from front to back being circular, the sides, 45, and », are flat, and a projecting base, 45, extends below it. This bone, 45, », entering between the ankles, 44, * and forms the articulation of the foot with the leg, or v hat is called the FOOT. 73 ankle-joint. Tliis bone, which is called the astragalus, has, therefore, the motion of a hinge between the ankles. Fig. 45. Fig. 46. Under the astragalus, extending to some distance before and behind it, are six other bones, the principal of which is that of the heel, 45, ' ; another, 45, sl, is placed under, and a little in front of the astra- galus ; and the four others, 45, s:i, -'4, ss, and 15, are placed between these and the five metacarpal bones, which extend along the instep to the origin of the toes. The form and arrangement of these latter are peculiar, and worthy of attention. They form an arch, being curved both laterally and forward, so that a line drawn over the surface of the foot from the instep to the origin of the great toe would, in a well-formed foot, be convex upwards, and a line drawn across the foot from the inside to the outside would also be convex upwards. But its convexity would be turned rather outwards, commencing at an elevated point on the inside, and extending to the point where the sole touches the ground on the outside. Tliis arched or dome-like form of the bones of the foot is attended with several mechanical advantages, among which may be mentioned the protection it affords to the vessels and nerves which pass from the leg to the foot, and the firmness and elas- ticity which it gives to the foot, both as the basis of support for the weight of the body, and as its principal instrument of locomotion. The ligaments by which the upper extremities of the bones of 74 ANIMAL PHYSICS. the leg are connected with the lower extremity of that of the thigh, are shown by a front view of the left knee in fig. 47, and by a back view in fig. 48. As in the case of the other principal articulations, these ligaments Fig. 47. Fig. 4S. completely envelope the joint. They are, however, classed by anatomists as four ; the lateral being 47, 2,3, the posterior, 4S,5, and the anterior, called the ligament of the ‘patella, fig. 47, *. Besides these, the bones are con- nected by two other ligaments which are concealed in the figures, and are called the crucial or oblique ligaments of the knee. The two bones composing the leg are connected together by an inter- osseous membrane, 49, 2, and anterior ligament, 49, ', and a similar posterior one not seen in the figure. The ligaments connecting the bones of the ankle and foot are shown in fig. 50, where the sole is presented to the observer. Each of the seven bones composing the instep are tied together by independent ligaments, and the metacarpal bones and phalanges of the toes are connected by ligaments so similar to those of the hand that they need no special description. A side view of the inner ankle is given in fig. 51, and of the outer in fig. 52. The foot is here shown to be connected with the leg by four ligaments, one (51, 6) extending from tlie inner ankle to the heel-bone and laterally over the instep at 51,', aud backwards towards the heel at 51, 10. The tendon, 51, s, extending upwards towards the calf of the leg is that well known as the tendon of Achilles. The anterior and posterior limits of the astragalus are shown in 51, s. The three other ligaments (52, fi, 52, ", and 52, s) have their origin in the outer ankle (52, 2), the first being distinguished as the anterior, the second as the middle, and the third as the posterior. The anterior and posterior ligaments are inserted in the anterior aud posterior parts of the astragalus, and the middle ligament in the heel-bone. The external FOOT, 75 Fig. 51. Fig. 52. 76 ANIMAL PHYSICS. ankle is also bound to the foot by a bundle of fibres spread over the instep (52, 9). 105. In comparing tlie mechanical properties of the ankle with the wrist, their adaptation to the respective functions of the foot and hand is very conspicuous. The organ of prehen- sion and touch requires to be presented with promptness and facility in all directions. We accordingly find it articulated, not to the ulna, which, being hinged upon the humerus at the elbow, is incapable of having that motion of rotation round its longitudinal axis which would be necessary to enable the hand to present itself in all directions — but to the radius, which, as already explained, is so articulated with the ulna, as to be capable of revolving round it, carrying with it the hand. This motion of rotation, given to it by the radius, combined with the hinge motion of the wrist upon the radius, gives to the wrist the mechanical properties of the universal joint. This mode of connection, perfect as it is for the purpose of prehen- sion, would, however, be incompatible with the mechanical con- ditions necessary for an organ of support and locomotion. We find accordingly, in the foot, which is not an instrument of prehension, but where the conditions of support and locomotion are indispensable, that the second and lesser bone of the leg, the fibula, which corresponds to the radius, unlike the latter, is articulated with the tibia, so as to be incapable of revolving round it. This permanent connection fixes the inner ankle, formed by the projection of the tibia, and the outer ankle, by the projection of the fibula. They, therefore, hold firmly between them, like the plates of a hinge, the circular pro- jection of the astragalus. They thus restrain the position of the foot laterally, so as to prevent it from turning on the ankle without a more or less violent strain. In short, the ankle thus acquires all the characters of a hinge-joint. The foot is inca- pable, therefore, of turning on the ankle inwards or outwards ; and since the knee is also a hinge-joint, the power which we possess to turn the foot horizontally round the longitudinal axis of the leg is derived entirely from the hip-joint. The form of the foot, as well as its position relatively to the leg, is admirably adapted to the support and locomotion of the body. In its normal position it is at light angles to the leg ; and, therefore, horizontal when the leg is vertical. It has con- siderable length from heel to toe, so that the two feet give a large base for the support of the body. Such a base is formed FOOT. 77 by the quadrilateral figure produced by drawing lines connect- ing the extremities of the toes and heels. The heels projecting backwards, the posterior side of this base of sustentation falls behind the line of direction of the centre of gravity of the body, as the line joining the toes forming its anterior side falls before it. Owing to the arched form in which the bones of the foot are arranged, the sole of the foot is not a uniformly flat surface, being concave under the inner ankle, so that its line of con- tact with the ground may be considered as a semi-circle, or rather a semi-ellipse, the convexity of which is presented out- wards, the extremities of the curve being the part of the heel which rests upon the ground, and the ball of the great toe. This form is attended with obvious advantages. If the sole of the foot were absolutely flat, it would be much more liable to injury by the inequalities of the ground, while it would afford no greater base of sustentation than is given by the semi-ellipti- cal line of contact just described. By the combined effects of the ankle, knee, and hip-joints, the centre of gravity of the body in the act of walking, or run- ning, advances forward in a line nearly horizontal. Without the flexibility of the knee and the hip, the body carried over the hinge-joint of the ankle would, in making each step, cause its centre of gravity to be moved in the arc of a circle, concave downwards. That centre would be raised in the first half of the step, until the leg would take the vertical position, and it would then fall. Thus, the action would consist of an alter- nate elevation of the whole weight of the body, succeeded by a jolt, or shock, produced in its descent. This, however, is prevented by the knee-joint bending slightly, as the body advances over the leg. 78 ANIMAL PHYSICS. CHAPTER III. THE MUSCLES. 106. In tlie animal economy, no organs occupy so large a space as the muscles. They constitute nearly the whole of what is commonly called flesh, being that part of the animal which, when used as food, is distinguished from the fatty parts by the term lean. It is to the conformation of the muscles that the body owes all the finer details of its external appearance. The bones, maintained in their proper relative positions by the liga- ments, confer upon the body only its general outline. But all its finer and more delicate details, and more especially those by which individuals of the same species are characterised and identified, are due to the peculiar form and development of the muscular system. We accordingly find no parts of the body vary so much with age and sex, with the sanitary state of the individual, and with personal occupation. No organs are so conspicuously affected by exercise, or the want of it. By gra- dually increased action, without being over-strained, the de- velopment of the muscles is most remarkable. Thus, they are frail and feeble in children, in females, and in all those who by habit or profession are speculative and sedentary ; while, on the other hand, they are conspicuously enlarged in all, who, being otherwise in good sanitary condition, are devoted to laborious pursuits. As may be expected, the particular muscles which are most called into play in each occupation, are those which are most developed. The arms of the blacksmith are like those of the statue of Hercules, though his legs may, at the same time, show the more delicate forms of the Apollo. The haunches, thighs, legs, and feet of a stage-dancer, exhibit a striking example of muscular force, the flesh being nearly as hard as bone, Avhile the arms are comparatively puny. 107. When the muscular power of individuals is, so to speak, pretematurally increased, whether in the case of the training of a race-horse, the practice of an opera-dancer, or the exercise of pupils under the calisthenic system, it is not enough that the exercises should be prosecuted gradually, but each day’s prac- GYMNASTIC EFFECTS. 79 tice must be preceded by a tiresome preparation. In gymnastic establishments, for example, the pupils begin by walking for a certain time round the arena, after which they run, first gently, and then more rapidly, until their muscular system is brought to that state of elasticity and vital energy, which is necessary for the safe display of the higher feats of strength and agility. Although this department of physical education is, when properly conducted, productive of incontestable advantages, it is one which, in the opinion of high medical and physiological authorities, is not unattended with danger. “ By such system of gymnastics,” observes Sir Charles Bell, ‘ 1 children are urged do feats of strength and activity, neither restrained by parental authority, nor even left to their own sense of pleasurable exertion. They are made to climb, to throw their limbs over a bar, to press their foot close to their hip, their knees close to their stomach, to hang by the arms and raise the body (thus reversing the natural action of all the muscles), to hang by the feet and knees, to struggle against each other by placing the sole of their feet in apposition, and to pull with their hands. No doubt, if such exercises be persevered in, the muscular powers will be strongly developed. But, the first question to be considered is, the safety of this practice. We have seen a professor of gymnastics, by such training, acquire great strength and prominence of muscles ; but by this unnatural increase of muscular power, he became ruptured on both sides. The same accident has happened to boys too suddenly introduced to such exercises.” 108. How necessary a gradually increased system of action is to the due development of muscular power, is proved by the treatment of horses, and especially those destined for the extra- ordinary exercise of racing. A person who receives a horse fresh from the stables of a dealer, is often surprised and disap- pointed to find that in a week or two he falls out of condition, and infers, often too inconsiderately, that he has been over- reached ; when, in fact, the cause of the animal’s decline of condition is merely the sudden transition from the quiescence of the dealer’s stable, accompanied by a regime suitable to it, to a state of regular work and a different regime. The pur- chaser, in such cases, may consider himself fortunate if a temporary falling off of condition be the worst evil which ensues, inflammation and congestion of the lungs being a possible and not at all improbable consequence of such pro- ceedings. By a regulation of the army, when young horses 80 ANIMAL PHYSICS. are brought into a regiment, they are walked an hour a flay for the first week ; two hours for the second week ; and three hours for the third. They are then walked till they are fatigued, but not sweated ; and by continuing the same very gradual increase of exercise, are at length brought into regular working order. Race-horses, under three years old, are in like manner brought very slowly to their exercise, beginning with the lounge, after which a very light weight is put upon them, which is gradually increased to the prescribed limit. Nothing can show in a more striking point of view the effects of exercise in perfecting the muscular action, than the consequences which often arise from the loss of one day’s training. It will bring the favourite to the bottom of the list, and that without any suspicion of lame- ness, but from a knowledge of the fact acquired by experience, that even such a slight irregularity in his training will have a sensible effect upon his speed.* 109. Although the contraction of a muscle is necessarily succeeded by its relaxation, the muscle in that state of relaxation is, nevertheless, not absolutely inert or flaccid. The relaxation, therefore, which the muscles have in a state of repose, must be understood rather in a relative than absolute sense. They are always, whether acted on by the nerves or not, in a certain state of tension, the force exerted by each being equilibrated by a corresponding force of the antagonistic muscles. Thus, the parts of the body subject to voluntary motion may be under- stood, when in a state of repose, to be in a mechanical state similar to that of a lever which is loaded with weights, which are reciprocally as their distance from the fulcrum, the normal tension of the muscles connected with a member in a state of repose being necessarily subject to a like mechanical condition. It is supposed that this normal tension of the muscles, like the more intense contraction which results from the dictate of the will, is due to the stimulus of the nerves ; but this stimulus, like that produced by the great sympathetic system, is inde- pendent of the will. According to this doctrine, therefore, which appears to be generally accepted, the nerves, independent of the will, maintain a constant action on the muscles ; and the power of the will consists in either augmenting or diminishing the intensity of this action within certain limits. 110. The force with which a muscle contracts, other things being the same, will be proportional to the number of muscular * Bell, Animal Mechanics, p. 29. 81 MUSCULAR FORCE. fibres, and therefore to the area of the section made by a plane at right angles to the fibres. But, it will also obviously be proportionate to the individual force of the component fibres, and to their density, the last mentioned circumstances being dependent upon the state of nutrition of the muscle, and the perfection to which it has been brought by exercise. There is nothing, however, in the organisation of the body more truly wonderful than the precision with which the will acts upon the muscle. The force transmitted appears to be conveyed from the sensorium, separately, to each component fibre of the muscle ; and it is measured so nicely that the resultant of all these elementary forces, countless in number, has exactly the intensity which is desired. Nor are the conditions which determine the direction of the force less wonderful than those which fix its intensity. This direction is often determined by the combined action of two or more muscles, to each of which a force must be conveyed by the will, such that, when combined according to the mathe- matical principle of the composition of forces, these several components shall produce a resultant having the desired direc- tion and intensity. 111. A very erroneous estimate of the functions of the mus- cular system would be formed, if it was regarded as a mere apparatus for the production of motion: so far the contrary is the fact, that it may be stated with truth that the voluntary muscles are much more habitually in a state of statical than dynamical action, and we must not here be understood to imply merely that state of muscular tension in which the muscles exist when not under the immediate operation of the will. There is no position of the body, while we are awake, however stationary it may be, in which innumerable voluntary muscles are not in a state of statical action. If we stand erect, the muscles which hold the lower members in a state of extension, and those which support the spine and head in the erect position, must be in a state of voluntary tension. If we sit with the back unsupported, the same will be true of the vertebral column and the head. If the back alone be sup- ported, the same will be true of the head. All this will be most apparent, if we consider that, the moment the control of the will is suspended by sleep, all the joints become relaxed, the knees and hips sink, the spine bends forward, and the chin falls upon the breast. In fact, in such a state, all the movable parts of the body fall into that position into Ci 82 ANIMAL PHYSICS. which their gravity would bring them by the common laws of mechanics. 112. But as the statical tension of the muscles is as much an exertion of vital force as their dynamical contraction, it i- necessarily followed by a sense of fatigue and exhaustion, and requires intervals of repose. When a person stands erect, the muscles of the legs, back, and neck being in a state of tension, soon become fatigued. If he sit upon a bench without a back, those of the legs are relieved, but the dorsal and cervical muscles are still in action. If he sit on a chair having a back, the cervical muscles alone are in action ; and if the back of the chair be sufficiently high and otherwise adapted for the support of the head, the cervical muscles are partly, but not altogether, relieved, as is proved by the fact, that if he fall asleep in that position, the chin will fall upon the breast, showing that while awake it was supported by the voluntary tension of the muscles of the back of the neck. Even in the position here supposed, the muscles of the arms are not altogether in repose ; and, to give complete relief, the arm-chair has been invented. 113. It is not merely to the voluntary muscles that repose alternated with action is indispensable ; and it may therefore be asked, how such occasional repose is seemed to the involuntary muscles, these being permanently removed from the dominion of the will, and then’ action being indispensable to vitality, even during sleep. We find in this case another of those countless evidences of the infinite beneficence and wisdom with which the Creator of the universe has provided for the well-being of the most minute and insignificant, as well as of the most exalted, of the creatures which he has called into existence. All the functions which depend for their play upon the action of the involuntary muscles are essentially intermitting, so that, although these may in a certain sense be pronounced to be in a state of never ceasing action, from the moment at which animal vitality commences, imtil that at which it ceases, the tension of the muscidar fibres is not continuous, but is regularly altemated with intervals of relaxation, however short ; so that if the sum of all the intervals during which these muscles are relaxed in the whole duration of the life of an individual were taken, it would be found to be equal to the sum of the intervals of their tension. 114. Certain recent physiological researches have led to the discovery that the contraction of the muscles is not a uniform MUSCULAR SOUNDS. 83 mechanical effect resembling the tension of a spring or of a cord having a weight suspended to it ; but that it is an act compounded of an infinite succession of partial and momentary contractions, which are continually shifting their place on the muscular fibres. This fact, which seems to have been ascer- tained by direct observation by Mr. Bowman, is in remarkable accordance with a muscular phenomenon first noticed by Roger, and described by him in a work published at Gottingen, in 1760, but which Avas since much more fully investigated by 1 Dr. Wollaston. This phenomenon consists in a certain sound produced by the muscles in contracting, which Dr. Wollaston described as resembling the distant rolling of a carriage, and which Roger expressed by the Latin word susurrus (a low murmuring noise). This acoustic phenomenon is therefore ex- plicable, if the vibrating motion of the muscles in contracting, as described by Mr. Bowman, be admitted. 115. The Muscular Sounds, whose vibrations have been ascertained to take place at the rate of twenty to thirty per second, have recently been submitted to examination by scien- tific medical practitioners, and their variation has bedn shown to be so connected with the phases of certain maladies, that their observation with the stethoscope has been admitted as an important diagnosis in medicine. 116. Number of Muscles. — When the infinite variety of motions of which all the parts of the body are susceptible is considered, and when it is remembered that a single muscle can only produce motion by drawing nearer to each other its origin and insertion, it cannot but be expected that the muscles should be extremely numerous. We accordingly find them variously estimated by different authorities. Thus, Theile gives them as 346 ; Chaussier as 368 ; and Sir Charles Bell as 436. They may, therefore, be stated generally, in round numbers, at about 400. It might be supposed that this great discrepancy between different and equally eminent anatomical and physiological authorities must arise from an imperfect knowledge of the structure of the muscular system. Such, however, is not the case ; the difference being entirely in words, and not at all in things. If each muscle had a single point of origin, and a single point of insertion, no difference would have existed in their enumeration. But such is not the case. Muscles which o 2 84 ANIMAL PHYSICS. have a single origin often have two or more insertions, and those having a single insertion have two or more origins. Anatomists are not agreed, in such cases, whether these are to he regarded as single or as so many independent muscles. 117. Classification. — Although the nomenclature of the muscular system is extremely complicated, and perhaps need- lessly so, it is, nevertheless, very significant and expressive. Muscles are named and classed, either from their form, their position, or their action. Thus we have the deltoid, the dentelated, the rhomboidal, the square, the triangular, the scalene, the long, the oblique, &c. ; all which terms explain themselves, being obviously taken from the approximate form of the muscle. Then we have them grouped according to the region of the body in which they are placed : those of the face being called the facial; those of the arms the brachial ; those of the breast the pectoral ; those of the back the dorsal; those of the neck the cervical ; those of the hips and haunches the pelvic; those of the legs the crural ; and so on. 118. Nomenclature. — But the most important principle of the muscular nomenclature, at least for our present purpose, is that which depends on the action of the muscles. Thus, when a movable part is bent with a hinge motion, like the knee or the elbow, it is necessarily placed under the action of two muscles, one of which inflects the parts towards each other, and the other extends them into a straight hue. The muscle by whose contraction the former effect is produced is called the flexor, and that which produces the latter the extensor muscle. Thus the flexor muscle of the leg extends from the back of the thigh to the back of the leg, and the extensor muscle from the front of the thigh to the front of the leg, passing over the knee-pan. Muscles are also denominated according to the direction in which they move a member with relation to the trunk. Thus the muscle which moves the arm from the trunk is called abductor, and that which moves it to the trunk adductor. A muscle which lowers a member is called a depressor, and one which raises it a levator. A muscle which expands a part is called a dilator, and one which compresses it a compressor ; and so on. 119. The general symmetry which prevails in the structure of the animal body, and which has been already indicated in MUSCULAR NOMENCLATURE. 85 the distribution of the bones and their appendages on either side of the median plane, necessarily suggests a corresponding distribution of the muscles. Like the bones, they are therefore, with a few exceptions, formed in pairs, the individuals of each pair being precisely similar in form and magnitude, and having corresponding positions to the right and to the left of the median plane. The single muscles, which alone are exceptions to this, are so formed and placed that they are divided symmetrically into two halves, precisely similar to each other, by the median plane. 120. We have hitherto regarded the muscles merely as agents by which bones are moved ; their power is, however, equally extended to the softer parts of the system, on which their play is not less important than on the members of the skeleton. In the face, for example, where the play of the features is so varied and expressive, the only movable bone is that of the under-jaw ; and, consequently, with that exception, all the motions produced by the facial muscles are imparted to the softer parts. The involuntary muscles, generally, are inserted in the softer parts of the organs. 121. When the vast number of the muscles, and the great magnitude which must be given to many of them to confer on them the force necessary for the actions respectively assigned to them, are compared with the superficial extent of the regions they occupy, it will doubtless excite surprise how the space has been found requisite for a dynamical apparatus so compli- cated ; and although many, including all the involuntary muscles, are placed in the internal cavities of the trunk, the number which still remain to be located upon the external parts of the skeleton is far more considerable than would be sufficient to cover it. It has been so ordered, however, that the parts of the body in which the motions are most numerous are precisely those where least resistance is presented to tho moving power ; and, consequently, although the muscles in such regions are propor- tionally greater in number, they are smaller in magnitude, and therefore, collectively, occupy less surface. The face presents a remarkable illustration of this. On the contrary, those regions where the greatest resistance is opposed to the motions, and where therefore the muscles must be strong and large, present a much more extensive surface for their establishment. The trunk is a striking example of this. 8G ANIMAL PHYSICS. 122. But even this relation, which is maintained between the superficial extent of the several parts of the body, and the magnitude of the muscles with which they are invested, is not sufficient to allow these organs to be distributed superficially. They are therefore, everywhere, as well on the trunk as around the members, superposed in layers, the number of which is determined according to the proportion which the number and magnitude of the muscles bears to the superficial extent of the parts over which they are distributed. The number of layers varies in different regions of the body from one to six. 123. When the complication of this system of mechanical agents is considered, and when it is known that not only each particular muscle is subject to the immediate command of the will, but that, in many cases, different sets of fibres of the same muscle may be called into action at its dictates, sepa- rately or successively, and that to produce a single motion the combined action of many muscles is often required, the inten- sity of such forces being so nicely proportioned one to another that the force to be imparted to the member which they more shall be the exact mechanical resultant which such components would have according to the geometrical principles upon which the theory of the composition of force is based, — the promptitude with which springs so various receive and execute the dictates of the will cannot fail to excite the most profound sense of wonder and admiration. It is impossible to contemplate the performance of such mechanism, and the continuance of its action, without other derangement than such as may arise from external accidents during the life of the individual, without being impressed with a lively sense of the infinite inferiority of all artificial contrivances of human invention. 124. The face is the theatre of motions infinitely varied, which, though they include the play of the most important organs, are produced with the smallest amount of mechanical force. The tegumentary covering of the forehead and skull, the eyes, eyebrows, and eyelids, the nose and nostrils, the cheeks and temporal integuments, the lips, tongue, and chin, are severally susceptible of motions which give expression to the endless variety of sentiments, passions, and mental emotions, and which in each individual have characters so peculiar as to afford means of his immediate identification, and to distinguish him from all others of his kind. That such complicated mechanical effects should require an equally complicated system of moving powers, will excite uo surprise ; and we find, MUSCLES OP HEAD. 87 accordingly, that an apparatus consisting of about seventy pairs of muscles, spread over the head, face, and neck, is appro- priated to this purpose. So far as they can be rendered super- ficially visible by the removal of the skin and integuments, they are shown in fig. 53. A stratum consisting of five or six muscles (53, ’, ", 3, ', 6) of considerable surface, but little thickness, covers the entire surface of the head from the brows to the back of the neck, called by anatomists, according to their local position, occipital, frontal, and auricular, the action of which is to move the scalp, with the hair, the ears, the integuments of the forehead and temples, and the brows. By their contraction, the eyebrows are drawn upwards, the skin of the forehead thrown into transverse folds and wrinkles, the scalp and hair moved back- 88 ANIMAL PHYSICS. wards and forwards, and tlie features thereby made to express various and often opposite emotions, according to the greater or less extent to which the action of these muscles is called into play. Joy, surprise, astonishment, or ecstacy, are attended with, or expressed by, a certain elevation of the brows. The contractions and wrinkling of the forehead and the approach of the brows to each other, involve the more violent class of emotions, such as anger, hatred, indignation, and menace. The eyes and eyelids, with their appendages, are moved by not less than twelve pairs of muscles, of which, however, one only, called the orbicular (53, 6), is superficially visible. These govern the entire play of the eyeball and the eyelids, the flow and the suppression of tears, and, in part, the gestures of the brows. They combine, therefore, with the muscles above mentioned in the expression of anger and menace, and also assume the gestures which express the very different and opposite sentiments of tenderness, love, grief, mental pleasure, and anguish. The nose and nostrils are moved by six pairs of muscles, three only of which (53, ', 8, '') are superficially apparent ; and fifteen pairs are appropriated to the various motions of the lips, the chin, tlie cheeks, and the lower jaw. It will be observed that one of the most voluminous muscles, called the musseters (53, 1:), is appropriated — aided by another, not apparent superficially — to the motion of the lower jaw ; that motion being subject, in the act of mastication, to a greater amount of resistance than any other facial gesture. The motions of the neck, and consequently of the head, are subject to the action of about forty pairs of muscles, of which a small number only are superficially visible. And some of those which appear in the figure do not belong exclusively to the neck, but are shared between it and the trunk. Eight pair of muscles are more or less called into play to make the head incline forwards, among which is the long muscle (53, I9) extending from the ear to the point where the clavicle (53, 32) is ai-ticulated with the sternum, or breast-bone ; another, called the mylo-liyoidean, extending downwards from the jaw ; and another, the digastric (53, S1, 2;), extending from the inner extremity of the jaw on one side, and its outer extremity on the other, to the hyoid bone (53, Seven pair of muscles are employed, together or separately, in inclining the head backwards, among which there appear in the figure the trapezius (53, *") and the splenius (53, Seven MUSCLES OF TRUNK. 89 pair are engaged in inclining the head sidewards, several of which are also those, such as 53, lu and 53, '°, which incline the head backwards. 125. The trunk is surrounded by about a hundred pair of muscles, which are superposed in six layers upon the back, but less crowded in front ; a circumstance which will be easily understood when it is remembered that the centre of gravity of the trunk, being placed in front of the vertebral column, must necessarily be supported by the reaction of powerful muscles posterior to it. 126. Owing to this thick superposition of so many layers on the back, it is difficult to present by diagrams any distinct representation of the muscular structure of that region. Some notion of it, nevertheless, may be conveyed by exhibiting suc- cessively one layer after another. In fig. 54 the superficial muscles of the back, including the neck, shoulder, and haunch, are shown on the left side of the spine ; and those of the second layer, disclosed by the removal of the former, on the right side. It will be seen that the superficial muscles are few in number, and great in magnitude. Those of the back, properly so called, being only two ; the trapezius extending from the neck (a) to the spine of the scapula and clavicle (54, 3 4) and downwards along the spine at 54, 13 . The other, called the great dorsal (latissimus dorsi), (54, 9, Il‘) covers the remainder of the back and side, and extends upwards towards the arm-pit. A large angular-shaped muscle (54, 8), extending from the clavicle and the spine of the scapula (54, 3, 4) over the upper extremity of the arm, is called the deltoid from the resem- blance of its outline to the Greek letter delta, A, and another large muscle (54, u) covering the haunch is called the gluteus maximus. The muscular stratum beneath this is exposed on the right side of the figure, where the principal muscle in the dorsal region is the serrated or dentelated muscle (54, ,0. The action of the superficial dorsal muscles iufluences several motions of the body, and varies according as one part or another of their attachments become fixed by the dictate of the will. Thus, if the shoulders be held fixed, and both the trapezian muscles be contracted, the head will be drawn back- wards ; but if one be contracted and the other relaxed, it will be inclined to the side of that which is contracted. If, on the other hand, the head be fixed, these muscles act together or separately in elevating both or either of the shoulders, in which operation they are sometimes aided by the great serrated muscle (54, 1#). The great dorsal muscle, when it acts on the upper bone of the arm, draws it downwards ; and if the shoulder and arm be 90 ANIMAL PHYSICS. fixed, it reacts upon tlie ribs, and elevates them. Both these dorsal muscles occasionally react upon the spine, when the shoulder and arm become relatively fixed. Thus, if a man Fig. 54. walking on the edge of a raised footpath happens to incline a little on the outside, by a violent exertion of the muscles just mentioned, the spine is drawn to the opposite side, and stability restored. Exhibitors on the tight-rope have these great superficial muscles of the back in constant play, these being DORSAL MUSCLES. 91 the principal means of redressing their balance, and, by their alternate contraction and relaxation, causing the centre of gravity of the body to oscillate slightly to the right and left of a vertical plane, through the centre of the rope. The principal muscles of the third layer, as well as those of the second, being shown in fig. 54, on the right side of the spine, those of the fourth layer, disclosed by their removal, are shown in fig. 55. 21 Fig. 50. The muscles of these deeper layers act chiefly upon the trunk, their tension equilibrating with the weight of the body, which tends to bend the spinal column forward. Most of them are 92 ANIMAL PHYSICS. attached to the spinous and transverse processes of the dorsal and lumbar vertebra;. They maintain the trunk erect, whether we are stationary or in motion. If their contraction be extreme, the spinal column will be bent somewhat backwards ; which always happens when the natural weight of the trunk in front of the spine is augmented by a weight suspended in front from the neck. Thus, it may be observed, that when a hawker carries a basket or tray before him, supported by a strap passing round his neck, he assumes an attitude in which the vertebral column is bent backwards. If, on the other hand, a porter carry a load suspended from the shoulders upon the back, the muscles in front of the spine, antagonistic to the former, are called into action, and the spine is bent forwards. In such case the object aimed at, and attained by the gesture, is to bring the common centre of gravity of the body and its load over the base of sustentation. By the same reasoning will be explained the peculiar attitude of the body assumed by corpulent persons. In such a state the weight of the body is not only augmented, but, owing to the increased prominence of the abdominal region, the centre of gravity is thrown farther forwards, so that, to bring it back within the base of sustentation, the vertebral column must be bent more or less backwards. Such a person would find it difficult, or perhaps impracticable, to carry a load suspended in front from the neck, while one suspended from the shoulders upon the back would tend rather to relieve the dorsal muscles, by allowing the spine to assume the vertical position, the centre of gravity of the body and its load being within the base of sustentation. By providing various muscles which have certain mechanical functions in common, or are, as anatomists say, congenerate, Nature has wisely provided for the continuance of muscular action without over-fatigue. The benefit of this provision is especially conspicuous in the play of the deeper class of dorsal muscles now referred to. As these must necessarily support the vertebral column, whether we stand or sit, are in motion or at rest, they would be in constaut action, and would conse- quently soon be disabled by fatigue, were it not that congene- rate muscles, and even different parts of the same muscle, relieve each other. Thus, for example, the lower fibres of the spinal muscles, which pass from the sacrum to the processes of the lumbar vertebrae, aid the other muscles in the support of the column, by rendering the transverse processes to which DORSAL MUSCLES. 93 they are attached so many fixed points, from which the succeed- ing parts of the spinal muscles transmit their effects through the entire height of the column, by a succession of efforts pro- pagated upwards, with a sort of vermicular motion. When by this means the action of one set of fibres succeeds that of another, each will have its alternate intervals of contraction and relaxation, as well as the fibres of other muscles, in which this state of intermission is more manifest. The muscle called the scicro-lumbalis can draw down the lower ribs ; and, if the effort be continued, this influence must soon be propagated to the vertebral column, which is thus bent towards the side by means of the intimate connection between the heads of the ribs and the vertebrae. The lower fibres of the spinal muscles will produce the same effect, so that the action of these may in like manner be interchanged and intermitted. The spine admits, to some extent, of a motion of rotation, or, more properly, of a limited angular motion round its longitu- dinal and vertical axis. Thus, the head may be turned round horizontally, until the face looks over either shoulder, after which the vertebral column may be made to turn upon its own axis, until the face shall have completed almost a semicircle from the point at which its motion began, the pelvis and legs remaining, meanwhile, unmoved. The latter movement is effected by that peculiar action of the spinal muscles above referred to. But it is the muscle of the side opposite to that towards which the motion takes place whicli produces the rotation.* 127. The principal anterior muscles of the trunk and shoul- ders are shown in fig. 56, those on the left being superficial, and those on the right the deeper layer covered by the former. In proportion to the surface over which they are spread, these muscles are much less numerous than those of the back, a circumstance which naturally arises from the fact already indi- cated, that the weight of the trunk being chiefly in front of the spine, is altogether supported, and, for the most part, moved by the posterior muscles shown in fig. 54. The anterior muscles shown in fig. 56, though located upon the trunk, act, for the most part, in moving the arms and shoulders. The superficial ones on the left side of fig. 56 are few in number, and great in extent. The great pectoral muscle (56, 2 ,3) has its origin along the edge of the breast- bone (56, ')« and along something less than half the length of the clavicle (56, 6) and from these lines the fibres converge to a point a little below the * Quain's Anatomy, 5th Edition, p. 513. 94 ANIMAL PHYSICS. head of the humerus, and on the inside part of tliat bone. The clarieula! fibres of this muscle, therefore, draw the arm obliquely upwards and inwards, having a tendency to secure the head of the bone in its socket ; while the sternal fibres, being nearly horizontal, draw it inwards towards the side. Fig. 5G. The lesser pectoral muscle, which is covered by the greater, is shown at 56, 13. It is attached at its origin to three of the ribs,- — the third, fourth, and fifth, and at its insertion to a process (the coracoid) of the scapula. The dentelated insertion of the great serrated muscle already described, attached to the ribs, is shown at 56, 8. The front part of the deltoid, already mentioned, is also shown at 56, s. 128. Brachial Muscles. — The power of the great pectoral. 56, - and 3, upon the motions of the arm are considerable. Besides depressing the humerus, and drawing it into close contact with the side, its lower fibres will trail the arm along BRACHIAL MUSCLES. 95 the side and front of the chest. The lesser pectoral, 56, 13, draws the point of the shoulder downwards and inwards towards the thorax ; but the action of both of these muscles may be entirely changed if the points of their insertion be fixed. Those of their origin will then become movable ; thus, if the arms and shoulders be fixed, the muscles in contracting, pulling upon their points of origin on the ribs, will raise these bones, and thus enlarge the cavity of the chest. An example of tins is often seen in persons afflicted "with asthma, who, when they feel the want of a full inspiration, seize hold of some object, so as forcibly to fix the shoulders and arms, and thus to throw the whole force of the pair of lesser pectorals, 56, l:i, upon the ribs. When the shoulder-blade is fixed by the tension of the trapezius and rhomboid muscles already described, the great serrated, 56, 8, having its force thrown upon the ribs, acts upon them in the same manner as the lesser pectoral muscles just described. In this effort the shoulder is elevated, just as if a burden were sustained upon it. This exertion, however, can only be continued so long as the chest is kept inflated ; for if the air contained in it be permitted to escape, the external pressure of the atmosphere will be too great for the force of the muscles, and, in spite of their contraction, the chest will collapse and the ribs and shoulders descend. During all muscular ef- forts, therefore, which re- quire an elevated position of the shoulder and ribs, the chest must be kept well inflated. The necessity for this was shown by an inge- nious experiment made by M. Bourdon. He opened the trachea or larynx of a dog that had been in the habit of jumping and tum- bling at command, after which all the attempts of the animal to make such evolutions were unavailing, — the air which had been inspired into the chest escaping through the aperture thus artificially made. But when the 96 ANIMAL PHYSICS. operator, by uniting the margins of the wound, closed the aper- ture, the lost power was instantly restored.* The deltoid muscle, half shown in figs. 54 and 56, and more fully exhibited in fig. 57, is one of the most important parts of the mechanism of the arm. Its origin is extended over about a third (57, 2) of the clavicle to near the point where that bone articulates with the scapula, and also along the spine of that bone (57, 5). The fibres converge from this line to a point (57, *') upon the humerus at some distance below its head ; and since, according to what has been explained generally, the will has the power of directing its mandates to the. fibres, either collectively or separately, it can impart very different motions to the arm, according as the contraction is limited to the internal and clavicular fibres, or to the external or scapular, or, in fine, imparted simultaneously to all. It must be observed that the mechanical power exerted by the fibres of this muscle upon the ami is greatly augmented by their being carried over the head of the bone. The humerus is also connected with the shoulder and some other im- Fig. 5S. portant muscles, which are imperfectly seen in figs. 54 and 56, hut more clearly visible in fig. 58. From the point where the humerus is articulated with the scapula, a ridge of bone (58, >) called the spine of the scapula runs horizontally, and two muscles called, from their position, the supra- and infraspinatus muscles (58, 2 and 3), connect the humerus at points somewhat beyond its head with the inner edge of the scapula. Two other muscles (58, 4 and °1, called the teres minor and major, or lesser and greater round muscles, connect the scapula also with the humerus at points a little lower. * Bourdon, JIdmoires suv les Efforts, quoted by Quain. BRACHIAL MUSCLES. 97 The action of this set of muscles upon the arm is very various and important. By the collective action of all its fibres, the deltoid (fig. 57,1) can raise the arm directly from the side, so as to extend it horizontally at right angles to the body. It can then be alternately directed forwards or backwards by the contraction and relaxation of the anterior and posterior fibres of the muscle, assisted, however, in the one case by the lower fibres of the great pectoral 56,', and in the other by the teres major, 58, °. The force of the deltoid, owing to the mass of its fibres, is so great, that by a further contraction it can make the head of the humerus glide upon the cavity of the scapula so as to direct the arm vertically upwards. In this opera- tion the supra-spinatus muscle, 58, 3, plays an important part ; for, owing to the extremely shallow cavity in which the head of the humerus is articulated, there would be great danger of dislocation by such a vertical movement of the arm as that here described. This is, however, prevented by the supra- spinatus, which, contracting with great force at the moment of the elevation of the arm. retains the head of the humerus in its place, and discharges for the moment the functions of a ligament. The muscles represented in fig. 58, except the teres major 58, 5, are capable of rotating the arm externally ; their power being, like that of the deltoid, increased by passing over the bead of the humerus, and also by being attached to processes of that bone, which gives them a certain leverage upon it. The teres major, 58, 5, depresses the arm if it be raised, and also rotates it on its axis. If the arm be fixed, and the elbow removed from the side, this muscle, assisted by another called the triceps, which will presently be described, can draw the lower edge of the scapula, and with it the whole trunk, towards the arm. A front view of the humerus and left shoulder, divested of the deltoid muscle, is shown in fig. 59. The teres major, already described, is shown at 59, s, and the muscle called the sub-scapular, having its origin extended vertically along the surface of the scapula and its insertion upon the humerus a little below the head, is shown at 59, *. 129. The muscles which extend along the front of the humerus from the shoulder to the elbow, and have their inser- tion below the elbow on the bones of the fore-arm, appear also in fig. 59. The corresponding muscles on the back of the same bone, — the tendons of which, extending over the elbow, are inserted in the back of the fore-ann, — are shown in fig. 60. The principal muscle in front, called the biceps from having two origins, □ 98 ANIMAL PHYSICS. appears at 59, 7. Its origins, as indicated in the figure, are at tw o points not far removed from each other upon the scapula ; and its tendon, commencing a little above the hollow of the arm, passes under the muscles of the fore-arm, and is inserted in a small process issuing from the radius. Immediately within the upper part of this biceps muscle is one called the bracliialis anticus, or internal brachial muscle, 59,°, which extends along the lower half of the arm, being covered by the biceps. Its origin is in the front of the humerus below the deltoid ; and, passing before the bend of the elbow, it is inserted in a process of the ulna. Thus, it appears that while the biceps acts upon the radius, the internal brachial controls the ulna. Only one muscle, called the triceps cubiti, fig. 60, ', 2, n, lies upon the back of the humerus. As its name implies, it has three heads or origins. The first, the long part, 60, 2, is attached to the border of the scapula immediately below the cavity of the shoulder-joint. The second, the external part, 60, ’, is attached to the external and upper surface of the humerus ; and its internal part, 60, 3, to the posterior surface of the upper part of the humerus. The lower extremity of this muscle is attached to Fig. 59. Fig. 00. the part, 60, **, of the ulna willed the olecranon, which, projecting from its upper end, forms the point of the elbow. BRACHIAL MUSCLES. 99 When the origins and insertions of these several muscles are attended to, their mechanical action will be very apparent. The shoulder and humerus being fixed, the biceps 59, 7, and the brachialis anticus 59, '■> acting upon the points of the radius and ulna already described, draw them forwards, so as to incline the fore-arm at an angle with the humerus. These are, therefore, the flexor muscles of the fore-arm. On the other hand, the triceps 60,1, 60, 2, 60, 3, acting on the projecting bone of the elbow 60, 4, which is part of the ulna, has a tendency to straighten the arm, if inclined, and is, therefore, the immediate antagonist of the former. The flexor muscles are more numerous and much more voluminous than the triceps or extensor, and a greater leverage is moreover given to them, by being inserted at lower points, and with longer processes, on the two bones of the fore-arm. This arrangement is in obvious accordance with the mechanical exigencies of the member. When we use the arm to raise a weight or overcome a resistance, the object is attained always by inclining the fore-arm towards the shoulder ; and, even when no weight or resistance is opposed to the flexion, the weight of the fore-arm and the hand must be overcome. But whenever the arm is extended, not only is no force or resistance opposed to its extension, but the weight of the fore-arm and hand assists in producing the motion. Thus we find that a single extensor muscle is placed at the back of the arm with a very small leverage to produce this effect, the small leverage being at- tended with the advantage of straightening the arm with great promptitude. If the arm be felt, it will be found that the fleshy mass between the shoulder and elbow, in front, is much greater than that which is posterior, showing the much greater volume of the flexor compared with the extensor muscles. If the fore-arm be fixed by grasping any stationary object with the hands, the origin and insertion of the flexor muscles interchanging characters, their action will chaw the humerus, and with it the shoulder and trunk, towards the fore- arm, as is so often exhibited in climbing and various gym- nastic evolutions. If the humerus and fore-arm be both fixed, the scapula and parts connected with it will be moved by these muscles. 1 30. The muscles of the fore-arm are necessarily numerous and complicated, having under their control the infinitely varied motions of the hand upon the wrist, a considerable share in those of the fingers, and, in certain cases, producing the flexion and extension of the fore-arm on the elbow-joint, in which case 100 ANIMAL PHYSICS. they co-operate with the corresponding muscles of the humerus. When, however, it is considered how large a share must be assigned to the hand in the superiority which man possesses over all other animals, including even those which immeasurably exceed him in strength and swiftness, it must be admitted that its mechanism is deserving of more than common attention, even in the briefest sketch of animal physics. The bones of the fore-arm are surrounded by nineteen muscles : the bodies or bellies, as they are technically called, of which, being placed chiefly in the upper-half, give that peculiar taper- ing or fusiform shape to this part of the limb with which every eye is familiar, and which, 'when regular, as in the female arm. confers great beauty on the member. These muscles terminate above in single tendons, which are attached either to the humerus, a little above the elbow, or to one or other of the bones of the fore-arm, a little below it; but chiefly to the former, with which thirteen of them are connected. The bellies of these muscles being situated as just explained, round the upper part of the arm, the tendons which connect them with their origin are short ; but since the muscles, properly so called, terminate at a considerable distance above the wrist, their lower tendons, all of which descend to the wrist and many to the hand and fingers, are very long. This multi- tude of tendons, surrounding the wrist especially on the side of the palm of the hand, can be distinctly felt, there being no other fleshy matter at that part. Of these thirteen muscles, eight are placed in front of the arm, between the wrist and the bend of the elbow ; three are placed on the external border of the arm, between the thumb and the external comer of the elbow-joint ; and the remainder lie upon the back of the arm, between the back of the hand and the elbow. Those which lie on the front of the ami are the strongest and most voluminous, and give to the member the fleshy form found in that place. It will be remembered that in the normal position of the arm, supposing it to be held with the palm of the hand pre- sented forwards, the two bones of the fore-arm, called the radius and ulna, lie parallel to each other ; the radius being in the line of the thumb, and therefore outwards ; and the ulna in that of the little finger, and therefore inwards. It has been also shown, that the structure of the joints is such, that the lower end of the radius has the faculty of revolving round that of the ulna, and the hand, being attached to it. must participate MUSCLES WHICH MOVE THE HAND. 101 in this revolution. In this change of position, the radius ceases to be parallel to the ulna, and forms an angle with it, which continually increases until the palm of the hand is pre- sented backwards ; in which case, the lower end of the radius will have made half a revolution round the xdna. Of the two extreme positions of the hand, here described, that in which the palm is presented forwards, the radius being then in its natural position parallel to the ulna, is called the position of supination ; and that in which the palm is presented backwards, the radius being oblique to the ulna, is called the position of pronation. The muscles which move the radius round the ulna, so as to give the latter position, are accordingly called pronators ; those, which move it back to the hand, its natural position, being called supinators. 102 ANIMAL PHYSICS. The muscles which bencl the hand upon the wrist, so as to place the palm at an oblique or right angle, with the front of the fore-arm, are called flexors, and those which bend the hand backwards, so as to incline the back of the hand similarly to the back of the arm, extensors of the wrist. In like manner, the muscles which bend the fingers and thumb towards the palm are called flexors, and those which incline them in the contrary way, extensors of the fingers. These terms being understood, the arrangement and play of the muscles will be easily comprehended. A front view of the left arm and hand, the palm of the hand being supposed to be turned to the observer, and the thumb, therefore being outwards, and the little finger inwards, is MUSCLES OF FORE-ARM. 103 shown in fig. 61 ; where, however, the superficial muscles only are seen. A similar view of the same arm, with the superficial muscles removed, so as to disclose the deeper ones, is shown in fig. 62. A posterior view of the same arm, with the back of the hand presented to the observer, the superficial muscles only being visible, is shown in fig. 63 ; and a similar view of the same arm, the superficial muscles being removed, so as to disclose the deeper ones, is shown in fig. 64. Of the nineteen muscles above mentioned, two 61, 4, and 62, 6, are pronators ; and two others 61,13, and 64, 5, are supinators ; the former by their contraction turning the radius round the ulna, so as to present the palm of the hand backwards ; and the others drawing hack the radius to its former position, and presenting the palm of the hand forwards. The upper tendon of one of the pronators 61, 4, is implanted upon the inner corner of the humerus immediately above the elbow ; and the muscle, proceeding downwards diagonally, is inserted into the middle third of the radius. The other pronator is a much shorter muscle, passing across from the ulna at a little distance above the wrist to the radius above the thumb. Of the two supinators which are antagonists of these, one 61, 13, is attached above the elbow to the external comer of the humerus, and below to the lower end of the radius ; and the other 64, s, is also attached to the external comer of the humerus, and goes to the upper third of the radius. Of the remainder of the muscles of the fore-arm, six send down tendons which, passing the wrist, are implanted in one or other of the metacarpal hones, forming that part of the hand which is between the wrist and the upper row of knuckles. The others throw out divergent tendons, which descend to the phalanges of the fingers, some to the second, and some to the third. Those which proceed from the front muscles, passing to the palmar sides of the fingers, are flexors ; and those which proceed from the muscles on the back of the arm, going to the back of the fingers, are extensors. The muscles which lie along the external border of the arm, send their tendons to the wrist and thumb. One of these, inserted in the radius, is the principal supinator, and the others are extensor muscles of the wrist and thumb. It will be evident from considering tlie form of tbe lower part of the arm, the wrist, and hand, that unless some pro- vision were made to the contrary, the force of the brachial muscles, transmitted along their tendons to the various parts of the hand in which they are severally inserted, would neces- sarily cause the tendons, whether on the back or front of the wrist, to separate from it when the hand is inclined to the arm, and to take the direction of the base of a triangle of which the hand would be one side, and the ami the other. In assuming this position, they would obviously force the integument of the wrist outwards from its natural position. It is obvious 104 ANIMAL PHYSICS. that since this does not happen, some adequate mechanical pro- vision against it must be supplied in the mechanism of the wrist. Such an expedient is in fact found in two semicircular ligaments, which surround the wrist outside the tendons, one above the palm, and the other above the back of the hand ; the former being called the anterior, and the latter the posterior annular ligament. The latter is shown at G3, l5, the former being removed in fig. 61, for the purpose of showing more clearly the course of the tendons. These two ligaments, therefore, form a powerful bracelet, by which the numerous tendons running from the brachial muscles to the hand are bound to the wrist, whatever position the hand may assume. When the hand hangs in its natural position in the direction of the arm, the tendons produce no pressure on the annular ligaments ; but when the hand is bent inwards, the palmar tendons form an angle at the anterior ligament, which holds them down ; and when, on the contrary, the hand is inclined backwards, the posterior tendons form a like angle at the posterior ligament, by which they are held down. Synovial membranes are provided for the lubrication not only of these annular ligaments, but of all the grooves and surfaces along which the numerous tendons sent by the brachial muscles to the hand and fingers pass, so as to give the parts the utmost freedom of motion. The circumstance already mentioned, that not fewer than thirteen of the nineteen lower brachial muscles have their origin in the humerus above the elbow, will readily suggest the influence which these must have in certain cases, as flexor and extensor muscles of the fore-ann. When, for example, by the force of the muscles placed on the anterior surface, the hand is bent inwards as far as it can go towards the front of the arm, any further contraction of the anterior brachial muscles must inflect the fore-ann on the humerus ; for the hand no longer yielding to their action, the insertions of the tendons in the bones of the hand become fixed points, and the anterior annular ligament becomes a fixed pulley, by which the muscular reaction is directed upon the points where the upper tendons of the muscles are implanted in the humerus above the elbow. It is evident that, in this case, the contractile force of the muscles draws the annular ligament towards then- points of insertion in the humerus ; and siuco these points are above the elbow, the fore-arm will be bent, with more or less force, towards the humerus, and its muscles will thus become, for the moment, MUSCLES OP THE HAND. 105 congenerate with the ordinary flexor muscles which lie upon the humerus. 131. The Hand. — Besides the motion imparted to the hand by the brachial muscles, above described, there is another system of muscles established on the palm of the hand, which are more especially appropriated to the motion of the thumb and fingers. Four of these, the bellies of which form what is called the hall, are appropriated to the motions of the thumb. Four others, which form the thick fleshy mass on the inner border of the hand, are appi-opriated to the motions of the little finger, over which they are placed. The chief part of the muscles which move the other fingers, are the brachial muscles, already described, whose tendons pro- ceed from the fore-arm to the hand. There are, however, inter- osseous muscles which lie between the metacarpal bones, the action of which is to open and close the fingers, and which also have some slight effect in inflecting them upon the palm of the hand. The superficial muscles of the palm of the left hand are Fig. 65. Fig. 66. •ihown in fig. 65, and the deeper ones, disclosed by the removal of the former, in fig. 06. 106 ANIMAL PHYSICS. The dorsal interosseous muscles of the right hand, and their connections with the tendons of the long extensor muscles of the fingers, are shown in fig. 07, and the palmar, with their connections, in fig. 68. Fig. 67. Fig. 6S. The bracelet called the annular ligament, which retains the tendons upon the wrist, is only a part of a more extensive sys- tem of membranous binding, enveloping generally the muscles and their tendons. Wherever a considerable change of direc- tion takes place in the latter, as in the instance of the elbow and wrist, this membrane sometimes assumes the form of a strap or band. The tendons of the brachial muscles, after passing within the annular ligament of the wrist, pass along the hand, and most of them along the fingers. They are con- fined on the hand by a membrane such as that just described, and on the fingers by ligaments, which retain them in their position in the same manner as that in which the annular liga- ment of the wrist acts. Thus we may conceive the tendons and muscles of the hand and fingers retained in their position by being enclosed in a membranous and ligamentous glove ; and, in the same manner, those of the arm and humerus, by a mem- branous sleeve extending upwards from the superior edge of the annular ligament of the wrist. MUSCLES OF THE LEG. 107 132. Muscles of the Leg. — The muscular apparatus which moves the lower members has so close an analogy to that of the arms and hands, that it will admit of being rendered intel- ligible with much more brevity. The muscles by which the infinitely various motions of the thigh are performed, are inserted at their lower extremities in different parts of the thigh bone, but chiefly at points a little below its articulation with the hip ; and at their upper extre- mities, at various points of the pelvic bones and some of the lower vertebra:. The dynamical effect of each of these will depend on the position of them point of insertion in the thigh bone, the position of them point of origin on the pelvic bones or vertebra, and, in fine, on the course which the tendon takes in passing to the point of insertion. According as the point of insertion is on the outside, the inside, or anterior surface of the bone, the motion im- parted will be outwards, inwards, or forwards ; and according as it is nearer to or more distant from the articulation, the motion will be more or less prompt and rapid. 133. When the great play allowed to the thigh by the ball and socket character of its articula- tion with the hip is con- sidered, and when it is remembered that each particular muscle acting by itself upon a single tendon can only produce one motion, it may be expected that the muscles of the thigh must be very numerous. It is accordingly found that there are not fewer than sixteen connecting the thigh with the trunk, most of which are implanted in different parts of the pelvis, a few only having their origin in the vertebra. Since, in general, the object to be attained is to give promptitude and rapidity rather than force to the motions of the member, tho points of insertion in the thigh bone are, with a few exceptions, placed immediately Fig. 69. 108 ANIMAL PHYSICS. below tlie neck which connects the head of the bone with it- shaft. They are there inserted principally in two protuberances, one inside and the other outside, called trochanters, which are shown at 43 2, 3. By these projections the advantage of a certain leverage is given to the muscles. The more deeply seated muscles, connecting the pelvis -with this part of the thigh bone, are shown in fig. 69, which pre- sents a posterior view of the left hip-joint. The muscles which surround the thigh bone between the hip and the knee are shown in figs. 70 and 71 ; the former pre- senting a posterior, and the latter an anterior view of the left thigh. Fig. 70. Fig. 71. Of these muscles, 70', 70 -, 71 71 ", 71 3, 71", 71 l;, and 7l13, originate in the pelvis, have their insertions in the thigh bone, and are therefore motor muscles of the thigh. All MUSCLES OF THE LEG. 109 the others, passing below the knee, are inserted in one or other of the bones of the leg, of which accordingly they are either flexors or extensors, according as they are inserted in the pos- terior or anterior part. 134. The muscles which surround the bones of the leg, like those which invest the fore-arm, throw out long tendons, which, passing down the instep and to the heel, are inserted in the bones of the foot and toes in the same manner as those of the fore-arm are inserted in the bones of the hand and fingers. And in the same manner as the bellies of the muscles of the arm form the fleshy mass at its upper part, tapering into mere tendons at the wrist, the bellies of the muscles of the leg form the fleshy part of the calf, tapering as they descend into tendons which surround the instep and heel. F'g- 72. Fig. 73. A front view of the superficial muscles of the left leg is given in fig. 72, and a back view in fig. 73. 110 ANIMAL PHYSICS. The number of muscles which thus surround the leg is fourteen : seven in the anterior, and seven in the posterior part. Their tendons are con- nected, some with the bone of the heel ; others, with the bones of the instep ; others, with the phalanges of the great toe ; and others throwing out several divergent tendons, which are inserted in the phalanges of all the lesser toes. This tendinous connection of the crural muscles with the bones of the foot and toes is shown in fig. 72. The calf of the leg is formed by the combination of the bellies of three muscles, two of which are connected by superior tendons with the thigh bone, and the other with the bones of the leg. The tendons of these muscles, coalescing towards the ankle, form a single tendon, 73, 6, which is inserted below in the extremity of the heel bone. This, which is by much the thickest and strongest tendon in the body, measuring about six inches in length, is that known as the tendon of A chilles, so called from the well known Homeric fable of its being the only vulnerable part of that warrior. The great fleshy mass of the calf of the leg 73, 4, and the voluminous muscle 70, ', upon the haunch or buttock, are among the anatomical peculiarities which distinguish man from the lower species. The action of the muscles of the lower member is even more various and complicated than those of the arm. The reaction of the brachial muscles on the trunk, when them insertions in the arm are rendered fixed, are rare and exceptional, being mani- fested only in strained and unusual exertions of the body, such as that of climbing or the evolutions practised in gymnastic exercises. But the reaction of the muscles of the leg upon the trunk is habitual and constant, being always exerted when the body is erect and stationary. In that case, the insertions of the femoral and crural muscles becoming fixed, their tension is thrown upon the several points where their tendons and aponeuroses are inserted in the pelvis and lower vertebrae. Them effect in that case is to act as so many straps, by which the trunk is bound to the summits of the pillars formed by the legs, and held erect. The flexion of the knee, when the body stands erect, is pre- vented by the force of the extensor muscles, which have their origin and insertion in the anterior parts of the bones of the thigh and leg ; and in the same manner the leg is kept erect upon the foot by an equilibi'ium maintained between the ten- sions of the extensor and flexor muscles of the heel and instep. Independently of these statical properties of the inverted action of the crural muscles, they have several important dy- namical functions. Thus, when the trunk is inclined forwards, as in the act of bowing or stooping, the muscles inserted in the thighs, and originating in the lower vertebra’, play an obvious part — the insertions in that case becoming the MUSCLES OF THE LEG. Ill fixed points. If the muscles of both thighs conspire in this action, the trunk inclines forwards ; if one only act, it inclines sideways. The transverse direction of several of the muscles connecting the hip with the thigh, as may be seen in figs. 69, 70, together with the great mechanical advantage afforded to them by the length and obliquity of the neck of the thigh bone, fig. 43, 4, enables them to act in giving rotation outwards to the thigh, and with it to the whole member. In this, their analogy to the muscles connecting the scapula with the humerus is very conspicuous. The muscles of the leg perform an important part in the actions of walking, running, and, still more, in the artificial exercises of dancing and gymnastics. In walking, the contrac- tion of the muscles forming the calf draws the heel upwards, and places the sole of the foot in the position of an inclined plane. The trunk is thus projected forwards, and falls, in fact, but is arrested by the other leg, which is advanced during the elevation of the foot just mentioned. The same action is performed by the muscles forming the calf of the other leg, and the body is thus alternately projected forwards by the elevation of the one or the other heel. Since the centre of gravity of the body is not placed vei-ti- cally over each foot, but between them, the force imparted to it by the elevation of the heels will not be directly forwards, but a little sidewards, the left heel throwing it a little to the right, and the right heel to the left. This causes the alternate inclination of the body right and left, which takes place in walking. In pei-sons having a tendency to corpulency, this lateral movement cannot be easily resisted, and they conse- quently have that particular gait called waddle. In persons having a wide pelvis, and legs short in proportion to the trunk, the pelvis, and with it the whole body, is turned alternately right and left in walking ; so that when the right leg is put forward, the chest and face are perceptibly turned to the left, and when the left leg is put forward, they are turned to the right. 135. The muscles of the foot resemble, in all respects, those of the hand, the sole corresponding to the palm ; and as all the muscles of the hand are collected on the palm, all those of the foot are collected on the sole. Their distribution on the sole is also similar exactly to the distribution of the muscles of the hand upon the palm. The muscles which move the great toe, 112 ANIMAL PHYSICS. ■which is the analogue of the thumb, are placed on the inside, and those which move the little toe, the analogue of the little finger, on the outside ; the centre of the sole being occupied chiefly by the tendons of the muscles of the leg, proceeding to the intermediate toes. The superficial muscles of the left foot are shown in fig. 74, and the deeper ones in fig. 75. All these muscles act upon the Fig. 74. rig. 75. phalanges of the toes, in the same manner as the palmar muscles act upon those of the fingers. In fine, a system of provisions for retaining the muscles and their tendons in their position, similar to that already described in the case of the arms and hands, is found in the leg and foot. An annular ligament, 72,2, similar to that which surrounds the wrist, surrounds the instep, retaining in their place the tendons of the crural muscles which pass under it to the foot, and having the property of a fixed pulley in changing the direc- tion of the force of these muscles transmitted along the ten- dons. A membranous coating, like that described in the case of the hand, invests the foot and toes, forming a sort MUSCLES OF THE FOOT. 113 of sock by which the muscles and tendons are held in their places ; and a similar web, forming a sort of stocking, invests the leg. It may be observed, generally, that all these confining liga- ments and membranes are supplied with a synovial apparatus by which their internal surfaces are lubricated, so as to give free play to the muscles and tendons within them. i 114 ANIMAL PHYSICS. CHAPTER IV. THE STRUCTURE OF THE LOWER ANIMALS. 136. The bony frame-work and muscular apparatus, as they exist in man, are reproduced with more or less variation and modi- fication in all the vertebrate animals. How large a part of animated nature is comprised in these maybe judged when it is stated that, besides all the species of mammifers or animals which suckle their young, they include every variety of birds, reptiles, amphibia, and fishes. In this region of nature the Creator has therefore worked upon one simple and uniform plan, modifying its proportions and details, and the number of some of its subordinate parts, to accommodate the structure to the external conditions in which each class and species is placed. Although the comparison of the physical organisation of man with that of the lower animals exhibits in a striking manner all that he has in common with them, it renders manifest, at the same time, those provisions which set him conspicuously apart from, and exalt him immeasurably above them ; and so profoundly impressed was the greatest of modem naturalists with this evidence of man’s superiority, that he maintained that, without taking into consideration the reasoning faculty, man ought to be classified, not as a species of vertebrate animals, but as an order apart, presenting the anomalous example of being the sole genus of his order and the sole species of his genus. * 137. Facial Angle. — Physiologists have traced a general relation between the degree of intelligence manifested by different organised beings, and the volume and structure of the brain, not only when species is compared with species, but when individual is compared with individual : and some have pretended to push this induction even so far as to connect different parts of the brain with different faculties, passions, * Cuvier. FACIAL ANGLE. 115 and tendencies, founding their conclusions partly on obser- vations of the brain in connection with the development of character, and partly on the analogies observable between the human brain, passions, and tendencies, and those of inferior animals. Hence has arisen that new branch of inquiry claiming a place in physiological science under the name of Phrenology. The proportion which the part of the head occupied by the principal organs of sense, — those of seeing, hearing, smelling, and tasting, — bears to the part occupied by the brain and its appendages, is found to be a good general modulus of the power of the intellectual faculties ; and accordingly methods have been sought by physiologists, by which this proportion can be conveniently ascertained with some degree of approximation by external indications, independently of the results of dissection. The method which has been most generally received is that pro- posed by Camper, an eminent Dutch naturalist, which consists in measuring what he called the facial angle, formed by a line, c d, a c b (fig. 76) drawn through the opening of the ear and the base of the nostrils, with another line, a b, drawn from the most salient point of the forehead, through the front of the upper jaw. This angle will be greater or less, according to the greater or less development of the brain, especially in its anterior part. In comparing man with the inferior animals, it is found accordingly, that his facial angle oxceeds those of the latter in a large proportion ; and in comparing different species of animals one with another, the variation of this angle is in remarkable accordance with their several manifestations of intelligence. i 2 Fig. 76. Fig. 77. 116 ANIMAL PHYSICS. The following are the facial angles of certain species, accord- ing to different physiological authorities : — Man (European) (fig. 76). . . . 85c to 90° Ourang-Outang (fig. 77) . . . . 56° to 60° Apes (fig. 78) 30° to 65° Dog 35° Ham 30° Horse 23° Fig. 78. Fig. 79. According to Professor Milne Edwards, the forehead in the case of a wild boar (fig. 7 9) is so formed, that it is impossible to Fig. 80. Crocodile. draw a straight line from the upper jaw to the most prominent part of the skull, the latter falling considerably behind the bony projection of the nose. a With birds and fishes the facial angle is less than with mam- FACIAL ANGLE. 117 mifers, and with reptiles (fig. 80) is often so small as to be scarcely appreciable. In comparing in- dividuals of the human race existing in different climates and under different physical influences, the facial angle is subject to much va- riation. Thus, while with the European (fig. 76) it is some- times so great as 90°, with the negro (fig. 81) it seldom exceeds 70°. The character which the facial angle imparts to the physiognomy is very apparent in com- paring, one with another, the several varieties of the hu- man race. Although the more complete investiga- tion of the connec- tion of cerebral development with the extent of the intellectual faculties was reserved for modem investiga- tors, it does not appear to have es- caped the notice of Fig. 83. (R. An.) Mongol. { Fig. 82 (R. An.) * European.) * The figures throughout this volume bearing this reference have been copied by permission of the authors and publisher, from the magnificent atlas of the R&gne Animal, by MM. Audouin, Blanchard, .Oeshaycs, d'Orbigny, Doyfere, Dugfes, Duvemoy, Laurillard, M. Edwards, Roulin, and Valenciennes, and published by M. Victor Masson, Paris, t From a Portrait of Suwarrow. 1 Portrait of a native of Kamchatka. 118 ANIMAL PHYSICS, the ancients, who evidently saw in the facial angle an index of intelligence. Not only do we find in their writings an erect frontal line noticed as a mark of a generous nature and an essential cha- racter of beauty, but the ancient sculp- tors conferred upon the figures of their heroes and their gods a facial angle much larger than is ever seen in man ; and in some of the more remarkable statues which have come down to us, — the Olympian Jupi- ter for example, — the frontal line b a, fig. 7 6, actually in- clines forward so as to render the facial angle obtuse. Even the most vulgar observation ascribes stupidity to a projecting mouth and nose and re- tiring forehead, to which the name muzzle is given, whe- ther found in men or in animals. And when in exceptional cases an apparent _. , v enlargement of the Negro. faclal anSle 1S Pr<> Fig. 84. (R. An.) Alfouroux.* » A Native of New Holland, from a Portrait of Ourou-SIare, a warrior of the Tribe of Gwea-Gul. QUADRUMANA. 119 duced by prominence of the bony arch -which protects the eyes, a spurious air of intelligence is produced, which causes qualities to be ascribed to animals having this conformation, which they do not really possess. QUADRUMANA. 138. It is found in the works of nature, as in those of art, that the more extensively the principle of the division of labour is carried out, the greater will be the perfection of the instruments. A tool or a machine, which attains two purposes, attains neither of them so perfectly as would two tools or machines especially adapted to the execution of each. Now we find, on comparing man with the inferior animals, that he supplies a solitary example of the rigorous application of the principle of the division of labour in the functions of his members. Its well-being requires that the creature should be supplied with members to seize and mem- bers to pursue the objects of its nutrition. Hence arises the necessity for members of prehension and members of locomotion. In some of the inferior animals, as, for example, certain quadrupeds, the four members are exclusively loco- motive, the act of prehension being confined to the mouth. In others, however, all the four members, besides fulfilling the functions of locomotion, are more or less prehensile, thus serving a double purpose, and therefore, according to the prin- ciple explained above, serving it by comparison less perfectly. In some, the prehensile functions of the four members prevail- ing over the locomotive functions, naturalists have given them the name of quadrumana, or form-handed, in contradistinction to that of quadrupeds, or four-footed, given to those species whose members are more exclusively locomotive. Of all the lower animals, this class includes those which come nearest to the human type, not only in mechanical structure and physical organisation, but also — though still at an immea- surable distance — in intellectual development. It includes all varieties of apes and monkeys, among which the ourang- outang approaches most closely to the human form, 139. Ourang-Outang. — To render this more evident, the skeleton of this animal is exhibited in fig. 80 ; the shaded parts surrounding the bones showing the general outline of the body. QUADRUMANA. 121 The names of the various bones, corresponding with those already given, in the case of the human skeleton, are indicated as follows ; and the same indications will be adhered to in other cases, which will occur hereafter. Fr. Frontal hone. Pa. Parietal. Te. Temporal. cY. Cervical Vertebne. St. Sternum, or breast- bone. Cl. Clavicle, or collar-bone. Sc. Scapula, or shoulder-blade. Rb. Ribs. dV. Dorsal vertebne. Hu. Humerus, or upper arm. 1Y. Lumbar vertebrae. Sa. Sacrum. Pe. Pelvis. Co. Coccyx. Fe. Femur, or thigh-bone. Ti. Tibia. Fi. Fibula. Ul. Ulna, or inner bone of fore- arm. Ra. Radius, or outer bone of fore- arm. Pat. Patella, or knee-cap. Ca. Carpus, or wrist. mC. Metacarpus, or palmar bones of hand. Ph. Phalanges, or fingers. P'h. Phalanges, or toes. Th. Thumb. Ta. Tarsus. mT. Metatarsus. It will be observed that the feet have the conformation and mechanical character of hands, the great toe being placed, like the thumb, in opposition to the other toes, which have the length, proportion, and structure of fingers. The beautiful application of the principle of the division of labour, observable in the structure of the human members, here disappears, with its consequences, the anterior members being as much feet as hands, and the posterior as much hands as feet. 140. Quadrumana Climbers. — This structure of the mem- bers, which gives an awkward and ungainly movement to them in ordinary locomotion, fits them in a peculiar manner for the act of climbing, in which all the four extremities, being equally prehensile, grasp the branches as they mount and leap from tree to tree. Fig. 87. Such a power is suitable to their organic wants. Like man, they are naturally frugivorous, their teeth consisting of incisors, canines, and molars. Upon the trees of the forest their food is produced ; and Nature has, accordingly, modified their members so as to give them every necessary facility for approaching it. Fig. 89. 141. Not Naturally Erect — From the mere view of the skeleton (fig. 86) of the ourang-outang, it might be imagined that the natural attitude of that animal would, like that of man, be erect. The comparison of the lower extremities with the human feet will, however, immediately dispel this error. Although it can, by a certain effort, walk upon its posterior extremities, the 122 ANIMAL PHYSICS. gait is awkward ; and the action cannot be long continued with- out the aid of a staff, which the animal is generally trained to use Fig. 87. THE OURANG-OUTANG. for the purposes of exhibition. Left to himself, he will always relieve himself by using his anterior extremities for partial support (fig. 88) ; and, to enable him to accomplish this without too great an inclination of the axis of the body, the extraordi- nary relative length, shown in the figure, is given to these members. 142. Prehensile TaiL— The double purpose of prehension and locomotion assigned to the members of the quadrumana, and their habitual exercise of climbing in pursuit of their food, and for protection from their enemies, render the occasional aid of another organ of prehension necessary ; such an organ is accordingly supplied them in the tail. In fig. S9 is represented the white-throated monkey thus exercising this prehensile action. The same action is common with the species called the Coaita, or spider-monkey, so named from the extraordinary length of its extremities, and from its motions. “ The tail,” says Sir Charles Bell, “answers all the purposes of a hand, and the animal QUADKUHANA. 123 throws itself about from branch to branch, sometimes swinging Fig. 88. (R. An.) by the foot, sometimes by the fore extremity, but oftener and with greater reach by the tail. The prehensile part of the tail is covered with skin only, forming an organ of touch as discri- minating as the proper extremities. The Caraya, or black howling monkey of Cumana, when shot, is found suspended by its tail round a branch. Naturalists have been so struck with this property of the tail of the Atdes, that they have compared it to the proboscis of the elephant, and have assured us that these monkeys fish with their tail. 124 ANIMAL PHYSICS. “ The most interesting use of the tail is seen in the opossum. The young of that animal mount upon her back, and entwine Fig. S9. (R. An.) THE WHITE-THROATED MONKEY. their tails round their mother’s tail, by which they sit secure while she escapes from her enemies.” * It will be observed that the young one, represented in fig. 90, also uses its tail as an organ of prehension, bolding itself upon the body of its mother by twining the tail round her. The varieties of these animals do not all exhibit the same close resemblance to the human form. Many have the appear- ance of quadrupeds ; some, such as the mandrill (fig. 91), resem- bling in their general form the dog. QUADRUPEDS. 143. While Nature has given to the tribes above referred to four members, more prehensile than locomotive, she has bestowed on a far more numerous class of animals four members which are either exclusively, or chiefly, locomotive and susten- * Bell on the Hand, p. 19. QUADRUPEDS, 125 tatory, from which character they have received the general denomination of quadrupeds. Fig. 90. (R. All.) THE KAMI. Fig. 91. (R. An.) THE MANDRILL. 126 ANIMAL PHYSICS. However striking may be the difference between these and man, the skeleton is composed of the same parts ■ the powers of locomotion peculiar to each species being produced, for the most part, by the mere variation of the relative lengths of the bones and some slight modifications of the joints. The general structure of the skeleton of quadrupeds may be illustrated by that of the camel, shown in fig. 92, the outline of the body surrounding the bones being, as before, indicated by the black shading. Fig. 92. (R. An.) The various bones corresponding with those of the human skeleton, are marked by the same letters as have been already adopted in the case of the ourang-outang (fig. 86). They need not, therefore, be again explained. In the present case, however, sV and c'V have been added, indicating the sacral and caudal vertebrae. How important a change has been produced by a mere variation of length of some principal, and decrease of number of some subordinate bones will be perceived by comparing the skeleton of the human hand and arm with the corresponding part of the fore-leg, as here shown. The humerus, or bone of the upper arm, which extends from the shoulder to the elbow, and in man is the longest bone of the arm, is here (Hu.) reduced in length, and buried in the flesh of the breast. The joint which corresponds to the human elbow is that which more immediately connects the summit of the QUADRUPEDS. 127 leg with the body. The bone (Ul.) of the leg extending from this joint to the knee of the animal is that which corresponds to the principal bone (the ulna), of the fore arm, and which extends from the elbow to the wrist. The radius is suppressed, or may be considered as cemented to the ulna, so that both form a single bone. The reason of this modification will be apparent if the peculiar function of the radius be taken into consideration. That bone, it will be remembered, constitutes the mechanism by which the hand is rendered capable of turning round the axis of the arm, so that the palm may be presented in any desired direction, without any change of position of the arm. This being a function peculiar to the prehensile character of the hand, and incompatible with the stability requisite for an instrument of sustentation and locomotion, it is consistently suppressed in the structure of animals, of which the anterior members are exclusively sustentatory and locomotive. The knee (Ca.) of the quadruped is the joint which corresponds to the human wrist ; and the bone (mC.), extending from the knee to the foot, is the analogue of the metacarpal bones of the hand, the number of which is here reduced to a single bone. In fine, the fingers are represented by the toes, the number of which is here reduced to two (Ph.). The hind legs of the quadruped correspond to the lower members of the human skeleton, and the comparison of them will produce like conse- quences. The thigh-bone (Fe.), the longest of those of the human skeleton, is here relatively short, and buried in the haunch. The joint which connects the hind leg with the body is that which corresponds to the human knee. The bone extending from this to the hock is that which corresponds to the bones of the leg, extending from the knee to the ankle. These bones, which in the human skeleton are two, the tibia and fibula, are here reduced, as in the case of the fore-leg compared with the arm, to a single bone which retains the name of the tibia, and may be regarded as the tibia and fibula soldered together. The hock (Ta .) of the quadruped is the analogue of the human ankle, and the bone connecting it with the foot, that of the metatarsal, or instep bones of the human foot, here soldered into a single bone. In fine, the plantar surface, or point of sustentation, consists, as before, of the toes or phalanges (P'h.) here reduced to two. 144. Analogy to the Human Form. — The analogy of the structure of the other parts of the skeleton to that of the bony frame-work of the human body will be obvious from the indi- cations on the figure without much detailed explanation. The head, being heavy and attached to the extremity of a long and flexible series of cervical vertebrae, and therefore requiring adequate support, a system of powerful muscles is provided by which it, as well as the neck to which it is attached, is connected with a series of long spinous processes issuing obliquely backwards from the dorsal vertebrae. On the other hand, the vertebral column not requiring the same power of inclination towards the abdominal region as it does in man, there are few or no flexor muscles corresponding with those which act on the human skeleton. 128 ANIMAL PHYSICS. The structure of the leg is subject to some variation in different quadrupeds, though retaining the analogies to the human members which have been indicated above. 145. Legs and Feet — When an animal is supported on four feet, the extent of its base of sustentation, and therefore its stability, cannot he augmented in any sensible degree by extending the magnitude of its plantar surfaces. The case is quite otherwise with bipeds, whose base of sustentation, other things being the same, will be in the direct ratio of the length of their feet. To have augmented the magnitude of the plantar surface of the feet of quadrupeds would have increased their weight, and dimi- nished their speed and activity, without con- ferring upon them any countervailing advantage. Nature, therefore, while she gave bipeds stability by making them walk on the soles of their feet, gave quadrupeds lightness and swiftness by mak- ing them walk on their toes. It is found, that in proportion as greater powers of speed are conferred on quadrupeds, the plantar surfaces of their feet and the number of their toes are diminished. Fig. 93. In fig. 93, is represented the lower part of the hind leg of the deer, the same letters of indication being retained as in the former figures. The lower end of the tibia, or leg -bone, is jointed with the metatarsal bone (mT.) at the hock (Ta.) which corresponds with the human ankle. The meta- tarsal bone (mT.) is articulated to the foot, which is flexible, and rendered highly elastic by its flexor and extensor muscles, and consists of a succession of three joints (Ph., P'h., P''h.), which correspond to those of the human toes, but are relatively larger, and possess a greater play of flexibility and extensibility. The toes are reduced in number to two, and the nails are developed into the cloven hoof characteristic of the animal. In fig. 94, is represented, in like manner, the hind leg of the horse, with like letters of indication. Here two rows of tarsal bones (Ta.) and (Ta'.) appear, as also a projection, t, representing the thumb in a rudimentary state. The toes are reduced to one, the nail of which is developed into the solid hoof. 146. Action of the Muscles. — The motion of the legs and the action of the muscles from which such motion imme- diately proceeds, are such as to impart to the centre of gravity of I QUADRUPEDS. 129 the body a series of impulses directed forwards and very slightly upwards ; the descent of the body between impulse and impulse being so regu- lated, that no injurious shock is produced when it is caught successively by the members put forward to receive it. Tn giving the impulse to the body the member acts like a spring which, after being bent, restores itself to its primitive form by virtue of its elasticity. The limb is bent at each of the joints by the action of the flexor muscles, so that each part is inclined to the parts -with wThich it is articulated at an angle more or less obtuse. The member, then, having its lower extremity firmly supported on the ground, straightens itself by the action of the extensor muscles, and in so doing projects the centre of gravity of the body forwards and slightly upwards. AVhile this takes place, the other member has been thrown forwards, and its lower extremity is planted upon the ground to give support to the weight of the body thus thrown forwards. In the case of man the connection of the thighs with the pelvis has an important relation to these movements. Had the hip-joint been made on a plan similar to the knee-joint, the action necessary to the progressive motion of the body would have been, if not impossible, at least awkward, ungainly, and inefficient. The peculiar articulation of the head of the thigh- bone with the external angle of the pelvic bone gives free play to the pelvis so that it can turn horizontally on the thigh- bone as an axis. When the legs are alternately advanced, this power is brought into play, the horizontal displacement of the pelvis in relation to the legs being more or less in different individuals. 147. Effect of Standing. — As has been already observed in reference to the human body, the attitude of standing at rest is far from being one in which the physical forces are inactive. The members, being all more or less flexible at the joints, would bend under the incumbent weight of the body if their rigidity were not maintained by the action of the extensor muscles. The continuance of such action absorbs an amorait of muscular K 130 ANIMAL PHYSICS. energy proportioned to the incumbent weight of the body, and to the length of the interval during which the action is con- tinued without intermission. Hence it arises that with animals, as with man, standing erect at rest is often more fatiguing than locomotion ; inas- much as in the former case the same set of muscles is kept in constant action, while in the latter different sets are brought alternately into play. 148. Action of the Hind-legs. — The power of quadrupeds to project their bodies forward depends much more on the action of the posterior than on that of the anterior members. The latter are chiefly useful for the support of the anterior part of the trunk, while its mass is projected forward by the posterior members. Strictly speaking, therefore, the anterior members ought perhaps to be denominated organs of support, rather than organs of locomotion. The fore-legs are alter- nately raised from the ground, it is true, and the feet are thrown forward ; but this is for the purpose of enabling the leg to catch the body as it falls, after being projected forwards and slightly upwards by the posterior members. 149. Bounding Animals: the Kangaroo. — In animals whose habits require a great bounding power, the members are constructed in accordance with the principles here explained. The posterior members have great proportional length, the anterior members being comparatively diminutive. By the pliability of its spine, and the flexibility of its posterior members, the animal can place itself preparatory to a bound so that, the lower bones of the leg being horizontal, the two superior bones shall be inclined to them at something less than a right angle, as shown in the case of the kangaroo (fig. 95) ; the profile and skeleton of which are represented in fig. 96. It may easily be perceived how powerful must be the bound which such an animal, or one such as the jerboa, or jumping-mouse (fig. 97), can make. A like structure is observable, though in a less exaggerated proportion, in the hare and rabbit, which run by bounds ; and in the cat and tiger, which pounce upon their prey. 150. These proportions are reversed in quadrupeds of slow locomotive powers, of which the girafle is one of the most remarkable examples. In this animal a great proportionate length is given to the fore-legs, so that, notwithstanding QUADRUPEDS, 131 Fig. 96. (R. An.) THE JERBOA, OR JUMPING-MOUSE. K 2 132 ANIMAL PHYSICS. tlie length of its neck, it would be incapable of taking its food from the surface upon which it stands. Nature has, however, beneficently adapted its wants to its structure ; and while she has elevated its head to a height of twenty feet above the ground, she has supplied it with nourishment at a corre- sponding elevation in the foliage of trees. The only known species of this animal inhabits Africa. 151. Fossil Quadrupeds : Cervus Megaceros. — The ani- Fig. 9S. Cervus Meoacep.os. mal remains which have been discovered in the researches of QUADRUPEDS. 133 geologists show that Nature, during those remote periods of time which preceded the present creation, worked upon the same general plan in the construction of animals as that which characterises the races which now inhabit the globe. Innumerable fossil remains of quadrupeds offer incontestable evidence of this. In fig. 98 is presented one of the most remarkable examples of fossil quadrupeds. This is a species of stag called by geologists the Oerous megaceros, found in the bogs of Ireland and in many other parts of Europe. The extreme spread of the horns of this animal measured ten feet. The analogy of the various parts of the skeleton to those already shown in fig. 92, is so obvious that it is not necessary here to particularise them. 152. Megatherium Cuvieri. — This animal, which is one of the most extraordinary results of geological research, belongs to a genus which has never existed upon the earth during the period of its habitation by the human race. The skeleton (fig. 99) was more than thirteen feet in length, and nearly ten in height. The haunches measured more than five Fig. 99. Megatherium Cuvieri. feet in breadth, dimensions which greatly exceed those of the corresponding bones of the largest existing elephants. Tho thigh bone was enormous, its diameter being more than half its 134 ANIMAL PHYSICS. length, a proportion without example in any living specie;-.. The head, like those of the varieties of the sloth, was small compared with the body, and the tail consisted of numerous vertebrse. The forms of its teeth proved that the animal was neither herbivorous nor carnivorous, but fed upon succulent roots, which it dug from the earth. Its prodigious tail served, in certain attitudes as an organ of sustentation, and probably also as an instrument of defence. The articulation of its fore- legs and the structure of the phalanges indicate the use of that member in digging for food. The body was supported by the hind legs and one foreleg, and partly perhaps by the tail, while the other foreleg was employed in digging. The great magnitude of the pelvis also favoured this action. The remains of this animal are found in the tertiary strata of the Pampas of Buenos Ayres, and in the caverns of Brazil. 153. Mylodon robustus — Owen (fig. 100). Like the me- Fig. 100. Mylodon Robustcs. gatherium, this genus, also extinct, resembles the sloth, and BIRDS. 135 is found in tlie same strata. Its dimensions are not nearly so great, but it differs in s tincture from the latter only in the character of its teeth, -which prove it to have been herbivorous, feeding on leaves and tender shoots. Not only the particular species of mammifers of which speci- mens have been given above, are limited to the tertiary rocks, but no remains whatever of any mammifers are found below them.* In these tertiary rocks, 115 different genera have been foimd, and these increase gradually in ascending. Thus, while the lowest stratum contains only 6, the next superior to it con- tains twenty-one, the next fifty-seven, and the upper stratum (the Parisian of the French geologists) seventy-two. When it is considered that these strata represent a series of successive periods in the chronology of the glohe anterior to the actual epoch, and that the number of living genera of mammifers is 210, it will he apparent that Nature, in the production of this her most perfect work of animal organisa- tion, has gone on progressively augmenting her operations ; and since man, the most perfect of all, has only appeared in the actual epoch (no human fossil being discovered in the tertiaries, still less in any of the older strata), it follows that while she augmented the number of her works she also exalted their excellence. Of the total number of fossil genera, sixty -four are extinct, and fifty-one are comprised among existing mammifers. BIRDS. 154. Animals which Fly. — Although, the conditions under which the inhabitants of the air and water live, and the move- ments they have to execute, differ so extremely from those to which we have hitherto referred, it is surprising by what apparently simple means nature has rendered the same general structure suitable to functions so different. Animals which walk, or otherwise move upon the surface of the ground, have a firm and unyielding support against which their organs of locomotion react, so that the whole muscular force which they exert is immediately absorbed by the momentum imparted to their bodies. In the case of animals which move through the air, however, so far from possessing such a fixed surface of reaction, the organs of locomotion have nothing whereon to support themselves or to act, except the lightest and most attenuated of natural fluids. The mechanical conditions of propulsion are therefore in this case wholly different. The organs of locomotion acting upon the air, impart to a certain * Remains have been found in two eases only in the middlo strata of tho Jurassic group, which have been conjectured to bo mammiferous. 136 ANIMAL PHYSICS. volume of it a moving force in a direction contrary to that in which the animal intends to move. By the general principle of the equality of action and reaction, the body of the animal receives an equal moving force, and if no impediment were presented, it would be moved forward with exactly as much force as that with which it had driven the air backward ; but its velocity would be just so much less than that with which the air is propelled backwards as the weight of the air thus propelled is less than the weight of the body of the animaL 155. Their Locomotive Apparatus. — But it is obvious that, in moving forward, the body of the animal passing through the air must displace so much of that fluid as is equal to its own volume, and consequently must impart to it a certain moving force ; and whatever be the amount of that moving force, it will be necessarily deducted from the force of propulsion which had been given to the body of the animal, the progressive motion of which will therefore be, not the whole moving force with which it drove the air backwards, but the difference between that force and the moving force which it gives to the air which lies in its way in passing through it. To render the locomotive power of the animal, therefore, as efficient as possible, two conditions should be fulfilled by its locomotive mechanism : first, such arrangement should be pro- vided as will enable it to propel backwards as great a volume of air as possible ; and secondly, its form and structure should be such as to displace as small a quantity of air as possible in moving forwards. We shall see presentty that in the structure of birds nature has fulfilled these mechanical conditions with the same admirable perfection which is observable in all her other works. The members which correspond to the arms and hands in men are the organs of flight of the inhabitants of Fig. 101. (R. An.) the air, and they are constructed accordingly, with modifications which render them suitable for this purpose. BIRDS. 137 156. The Bat. — There are some animals of the class of Fig. 102. (R. An.) mammifers which, exercising certain powers of flight, may in this respect be considered as holding an intermediate place between these and birds. Of this class, the bat (fig. 101) is an example, which is represented by its skeleton in fig. 102. To produce the organs of flight, in this case, the only change of struc- ture consists in increasing the proportionate length of the fore-arm (Ul.), and in a much greater degree, that of the fingers (Ph.) ; the thumb (Th.) being rudimentary and inactive. The fingers and arm are connected with each other, and with the legs, by an extensive web ; and by its muscular power, the animal being able to separate the fingers, this web becomes stretched like a sail, and thus re-acts upon the air. 157. Birds. — Although the skeleton of birds differs more than that of the bat from mammifers in general, it never- theless retains all the essential characters of the solid frame- work of vertebrated animals, the points in which it differs having chiefly reference to the powers of aerial locomotion. And here again we have reason to admire the very simple modifications by which the skeleton has been thus adapted. In figure 103 is represented the profile and skeleton of a vulture, and in fig. 104, that of a gull, the bones which correspond to those of mammifers being indicated on the figures respectively. 158. It will be evident by inspecting the indications on these figures, that the framework of the bird in all essential I>articulars is constructed upon the same general principles as 138 ANIMAL PHYSICS, Sc Hu Sa Fc cV Co Fig. 104. (R. Ail.) BIRDS. 139 that of man ancl other mammifers ; the adaptation of the hands and arms to the purposes of flight being accomplished by a mere modification in the proportion of the bones, and the reduction of the number of fingers. There are, however, some other points in the skeleton which will require notice. Since the organs of locomotion have not only to propel the animal, but also to support it when it has risen from the ground, it is essential that the weight of the body should bear the smallest practicable proportion to its volume. Hence we find that birds never attain to the great magnitude of mammifers, and that their bones and other parts of their system are so con- structed as to include considerable cavities, so as to render the body extremely light with relation to its bulk. The head is in general small ; and though composed of independent bones, corresponding in number and form Avith those of the human skull, they are only recognisable in the young, being so con- nected by ossification afterwards as to be undistinguishable. The mouth being, in many species, the only organ of pre- Fig. 105. (R, An.) THE CRANE. Fig. 10C. (R. An.) THE FLAMINGO. hension, the jaivs are formed into two oblong, pointed pieces, 140 ANIMAL PHYSICS. •coated with a horny covering, which constitute the bill, the form and magnitude of which is subject to much variation, being in every case adapted to the nature of the food on which the animal subsists. 150. The Neck and Head. — The articulation of the head with the vertebral column gives it much greater freedom of motion than is generally found in mammifers. The length of the neck, which is extremely flexible, is proportionate to the height of the body — a condition necessary to enable the animal ■with sufficient facility to pick up its food. In the case of certain aquatic birds which seek their food at a considerable depth in •the water upon which they float, a still greater length is given to the neck, as may be seen in the instance of geese and swans. In all cases, the cervical vertebrae are so articulated as to give the utmost possible freedom to the movement of the head; and this is especially the case with birds like the crane (fig. 105), and the flamingo (fig. 106), which, to seize their prey, must dart their bill with great rapidity to a con- siderable distance. La all such, numerous muscles are provided, attached to suitable processes in the cervical vertebrae, by the action of which these motions are imparted. 160. Dorsal Vertebrae. — While the cervical vertebras of birds are thus constructed with such perfect mobility, all the articu- lations of the dorsal vertebrae are, on the contrary, absolutely ossified, so as to render that part of the spinal column a single, strong, solid bone. 161. Birds which do not Fly. — The mechanical reason for this modification is as apparent as that which explains the ex- traordinary freedom of the neck. The scapula upon which the wing plays is attached to the ribs, and by them to the dorsal vertebrae ; and it is evident that the force by which the wing must react upon the ribs, and through them upon the dorsal part of the column, would be altogether incompatible with the flexibility given to that part of the column in mammifers. The dorsal vertebrae of the bird, therefore, are soldered together in ■order to give a firm point of reaction to the wings. That this is the true explanation of the rigidity of the dorsal part of the spinal column is rendered manifest by the fact that in the case •of birds, such as the ostrich (fig. 107), and the cassowary, (fig. 108), which never rise from the ground, and whose loco- motion is limited to walking and running, the dorsal vertebra1 have flexible articulations, like those of mammifers. BIKDS. 141 1G2. Locomotive runctions of the Tail — The vertebrae of the coccyx (figs. 103 and 104) are articulated so as to give them a certain play ; and the last of these has an increased magnitude and a projecting position, being the part to which the principal feathers of the tail are attached. The flexibility of the coccygian vertebrae has an important relation to the loco- motive powera of the bird ; the feathers of the tail, whose posi- tion they govern, acting as a sort of rudder in guiding the flight of the bird through the air. Fig. 107. (R. An.) SKELETON OF THE OSTRICH. 163. The Thorax. — It will be recollected that, in man and other mammifers, the ribs are connected in front with the breast-bone by certain cartilaginous straps. Now, it is evident that, in the case of birds, such a mode of connection would be quite incompatible with that solidity and firmness which is neces- sary to resist the action of the wings transmitted to the ribs through the scapula. We find, accordingly, that those parts of 142 ANIMAL PHYSICS. the ribs connected with the sternum which in mammifers are cartilage, in birds are bone. But there is another provision, still more admirable, to sup- ply a resisting power to the ribs proportionate to the vast force exerted in flight by the wings. To appreciate this force and the necessity for providing a suitable resistance against it, it must be considered that, when the expanded wing acts upon the an-, the centre of pressure is at a considerable distance Fig. 10S. (K. An.) THE CASSOWARY. from the articulation of the wing-bone with the scapula, while the insertion of the muscle which moves the wing is near the articulation. Nearly the whole of the reaction of the air upon the wing is thrown upon the scapula, by the scapula on the ribs, and by the ribs is distributed between the spinal column and the sternum. The ribs being semicircular, or semi- elliptical hoops, may be regarded as forming an arch, of which the spine and the sternum are the abutments, upon which, there- fore, the entire reaction of the wings must ultimately falL 1G4. The Breast-bone. — The form given to the breast-bone of birds, as compared with the sternum of mammifers, supplies another admirable example of the adaptation of means to an BIRDS. 143 end. Every one is familiar with. the form of the breast-bone of a fowl. It is a sort of inverted arch, not unlike the lower part of the hull of a ship ; and, like the hull, it is strengthened by a keel, which extends along the middle of its entire length. To the edges of this strong, bony, inverted arch, the ends of the ribs are firmly attached, not by articulations, but by solid ossified connections (fig. 104). In the same figure, it may be seen that, from the middle of each rib, a bony process issues, which is directed backwards, its end resting upon the preceding rib. There is thus a system of provisions for the firmness of the bony cage formed by the libs, not only at both extremities of each rib, but also in the centre. That the object to be attained by the keel erected along the middle of the breast-bone is to strengthen the trunk, so as to resist the action of the wings, is conclusively proved by the fact that with the ostrich and cassowaiy, which do not fly, there is no such keel. 165. The Clavicles. — There are, however, other modifica- tions in the skeleton of the bird which must be regarded as forming accessories to the machinery of flight, that cannot be passed without notice. It will be remembered that, in the human skeleton, the shoulders are kept apart by the clavicles, or collar-bones, which extend between them and the top of the breast-bone. But if the arms and hands, instead of being instruments of prehension, were organs of flight, the collar-bones, or clavicles, would be utterly insufficient to resist the reaction on the shoulders ; they would soon give way, the shoulders being dislocated and the wings disabled. In birds, therefore, the corner of the scapula, with which the wing is articulated, is connected with the breast-bone, not by one, but by two collar-bones, which, diverging at an angle, divide the reaction of the wing between two extreme corners of the sternum, giving the same Scapula. Fibrous membrane connecting the cla- vicles with the ster- num 1st. clavicle Keel of breast-bone 2nd. Clavicle or cora- coid bone. Sternal ribs. Sternum or breast- bone. Cavities in breast- bone. Fig. 109. support to the scapula, as would two flying buttresses, diverging at an 144 ANIMAL PHYSICS. angle of 45° from the corner of the structure which they are designed to support. To render this more clear, we have represented in fig. 109 the hones here referred to. It will be seen that the scapuhe which in man are tabular, are here long bones, which are parallel to the spinal column. The first clavicles on both sides approaching each other down- wards are united at their lower extremities with the front of the keel of the breast-bone. These two clavicles, therefore, looked at from the front, have the form of the letter Y, the coracoid bones forming flying buttresses, which, together with the diverging sides of the V, keep the shoulders apart, and supply to the wings solid and firm points of support. That the pur- poses designed by Nature in this mechanism are those here indicated, is proved by the fact, that in birds which fly but little, or not at all, the clavicles and coracoid bones are only feebly developed. Thus, in certain parrots of Australasia, these bones are altogether rudimentary. In ostriches and cassowaries they are generally, either small, or represented by a mere stylet, or point of bone. In fine, in certain owls, which fly but little, they are connected by cartilage. 166. The Wings. — Tlie power of flight depends, not only on the skeleton of the member, and the muscle which moves it, but also upon the extent of surface which is available for the displacement of the air. It will be remembered as we have already explained that, other things being the same, the power of propulsion will depend upon the volume of air displaced by the wing ; and it is obvious that this volume will itself depend on and be proportionate to the superficial magnitude of the ex- panded wing. But great superficial magnitude in such an in- strument would, unless expedients were provided against it, infer corresponding bulk and weight ; and such bulk and weight would operate with proportional force against the ascent of the animal. Expedients are provided, nevertheless, by which great ex- pansion of the wing and great power of resistance are rendered compatible with all the necessary lightness and with the mechanism necessary for -contracting the dimensions of the wing, so as not to impede the progress of the body between stroke and stroke. It is essential, in the first place, that the wing should extend to considerable length, measured from its articulation, and that its trans- verse strength to resist the reaction should increase gradually in approaching the articulation, and near to that point should be very considerable. And it is necessary, in fine, that these conditions should be fulfilled, without conferring undue weight upon the mem- ber. All this is admirably accomplished by the arrangement of quills and feathers with which every one is familiar. These quills are firmly inserted in the extreme phalange (fig. 104), which corresponds to the finger in man. But here the digital divisions disappear, not only being useless, but incompatible with the solidity necessary for the insertion BIRDS. 145 of the quills ; and the hand is reduced to a sort of flat and unarticulated stump. It is, however, in the structure of the quills, that the design of Nature is rendered most beautifully manifest. It is demonstrated in mechanics, that to render a rod most powerful to resist a transverse strain, without giving it undue weight, it should receive the shape of a hollow tube, the materials being as dense and compact as possible. Nature has, accordingly given this form to the part of the feather in the wing which requires most strength. At a certain distance from the articulation, the tube becomes a thinner rod, not hollow, or round, but square in its section, and filled with a sort of light pulpy matter, from either side of which issues a range of light feathers of beautiful microscopic structure, each of which has a certain power of resistance relatively to its size. These feathers are ranged side by side along the wing, increasing in length as they approach its extremity, so that the expanded wing has greater magnitude there, where magnitude has greatest mechanical effect (fig. 110). 167. Their Action. — The articulations of the wing and con- nection of the feathers are such that, by its muscular power, the bird, after expanding and making a stroke with it, can draw it back in such a position as to present only its edge to the air, and thus in advancing to displace the smallest possible quantity of air, and therefore to produce the smallest amount of resistance. In commencing its flight, it is first necessary to raise the body from the ground upwards, and the strokes of the wing are then nearly vertical, the air being driven moredownwards than backwards. But when thedesired elevation has been attained, and a progressive flight is required, the strokes of the wing are directed obliquely backwards. Nevertheless, as it is still necessary that the weight of the bird should be supported, the stroke of the wing can never be directly backwards, but must be oblique, with just so much of a downward direction as is necessary to balance the tendency of the body to fall by its weight. 168. To give effect to the action of the wings in flight, it is necessary to keep the centre of gravity of the body nearly under the point connecting the articulations of the wings, the axis of the trunk being horizontal. The form of the body of the bird is adapted to produce this efl’ect ; and by presenting its head forward, which it always does in flight, this position of the centre of gravity is further secured, and at the same time r. 146 ANIMAL PHYSICS. tlie resistance which would attend a different position of the head is avoided. It is evident that the power of flight, other things being the same, will be proportional to the magnitude of the wings ; and it is accordingly found, that all birds remarkable for rapid and long continued flight have large 'wings, while those whose wings are short relatively to the volume of their bodies, are Fig. 111. (R. An.) THE YELLOW VULTURE. Fig. 112. (R. An.) THE LAMB VULTURE (GypactuS Barba tUS). less swift of flight and require more frequent intervals of repose. 1 69. The birds most remarkable for flight, and consequently for the magnitude of their wings in proportion to their volume, BIRDS. 147 include different species of vultures, eagles, and the frigate- bird. Fig. 113. (R. An.) THE ROYAL EAGLE. 'ig. 114. (R. An.) the frigate-bird. ‘ °' The f°,ndor’ or vulture of the Andes, has wings , expanded, measure from point to point about fourteen l 2 148 ANIMAL PHYSICS. feet. It rises iiAo more elevated regions of the air than any other known bird ; sometimes at the level of the sea, it is seen at others floating above the summit of the Chimborazo at an elevation of 23000 feet. Its habitual dwelling is upon the crest of the Andes, immediately below the line of perpetual snow, at from 11000 to 16000 feet above the level of the sea. From these precipitous summits, it descends into the adjacent valleys and upon the plains to seek its food, which consists chiefly of the carcases of the large animals ; and it is even said that these enormous birds sometimes assemble several together, attack and kill animals, such as oxen, and have sufficient strength of wing to carry off in their talons entire sheep and lamas, and transport them to the elevated summits of the chain of the Andes. Although the frigate bird is less in magnitude, it is supplied with wings relatively larger, and has proportionally greater powers of flight. These birds, which inhabit the tropics, are sometimes seen at a distance of 1600 miles from land. 171. Classification of Birds. — Cuvier has resolved birds into six classes, distinguished by the forms of the beak and claws, as follows : — 1. The Birds of Prey (Accipitres, Linn.) : of which the beak is hooked, the nostrils being pierced through a membrane which covers the entire base BIRDS. 149 of the beak. The feet are supplied with strong claws ; most of these have a short web between the external phalanges. Fig. 115 represents the head and beak, and fig. 116 the claws, of the falcon (Falco biarmicus). 2. The Passerine Birds (Passeres) include more species than all the other families together, and vary much, both in magnitude and power. The two external phalanges are united at their base, and sometimes throughout Fig. 117. (R. An.) Fig, 118. (R. An.) a part of their length. Fig. 117 represents the head, and fig. 118 the foot, of the white-breasted thrush (Turdus torquatus). 3. The Climbers (Scansores) are distinguished by the feet, of which two Fig. 119. (R. An.) Fig. 120 ^ An>) phalanges are directed backwards. Fig. 119 represents the head, and fig. 120 the foot, of the Cape woodpecker (Picus capensis). 4. The Gallinaceous Birds (dallime): of which the domestic cock is the type. Fig. 121. (R. An.) The head is heavy, the flight imperfect and short, the beak of 150 ANIMAL PHYSICS. moderate size, the upper mandible being slightly bent. The nostrils are partly covered by a soft scale, and the phalanges dentilated at the edges. Fig. 122. (R. An.) having short webs between the bases of the front ones. Fig. 121 repre- sents the head, and fig. 122 the foot, of the common pheasant (Phasianus colchicus). 5. The Waders (Gralke, Linn.), which inhabit the banks of streams Fig. 123. (R. An.) and the shores of lakes and seas, have generally a small extent of web BIRDS. 151 between the phalanges, especially the two external ones. The tarsi are long, the legs denuded of feathers towards the base, and the body slender. In short, the form and structure of the bird is in every respect adapted to facilitate its characteristic locomotion, of wading through the water Fig. 124. (R. An.) in search of its food. Fig. 123 represents the head, and fig. 124 the foot, of a species of heron (Ardea coerulescens). 6. The Web-footecl Birds, or Water-fowl (Palmipedes), besides the complete junction of the phalanges by webs, are characterised by the backward position of the legs, the length of the sternum and of the neck, Fig. 125. (R. An.) and close, polished plumage, impermeable by water. Fig. 125 represents the head, and fig. 126 the foot, of the common duck (Anas boschas). 172. The Z.egs and Feet — .Birds not being always on the 152 ANIMAL PHYSICS. wing, and, when not so, becoming biped, nature has adapted Fig. 126. (R. An.) the structure of their legs and feet to the exigencies of their mode of life. In standing, a base of sustentation must be given to them of sufficient magnitude, and in such a position, as to keep the line of direction of the centre of gravity within it, without too fatiguing exertion of the muscles ; and since, as has been already shown, for the purposes of flight the centre of gravity has been placed immedi- ately below the line joining the ar- ticulations of the wings with the trunk, while the legs are generally placed towards the posterior part of the body, some expedient must be provided, by which the feet and arti- culations of the wings may be brought into the same vertical plane when the animal stands. This is ac- complished, partly by giving the leg such a structure that the tarsal bones, xue ibis. which extend from the foot to the body, have a sufficient length, and are inclined to the tibia or leg- bones (figs. 103, 104), so as to direct the foot forwards; while the body, on the other hand, assumes such a position, that the spinal column is inclined Fig. 127. (R. An.) BIRDS. 153- more or less upwards. The flexibility of the neck, which enables the animal to throw the head more or less backwards, aids in bringing the centre of gravity into the desired position, as shown in fig. 127 ; which represents the ibis. In the position of the bird here shown, the centre of gravity is thrown a little behind the line of articulation of the wings, and, therefore nearer the centre of the base of sustentation, by the backward position given to the head. When the animal flies, however, the feet are drawn up towards the breast, and the neck and head extended forwards in the direction of the spinal column, by which the centre of gravity is thrown forward, so as to come in to the position required for flight. 173. Standing. — In tlie case of birds, such, for example, as the penguin (fig. 128), having a short and nearly inflexible neck, and legs which are inca- pable of being advanced in the manner here described, the animal, when it stands, is obliged to assume the vertical position. 174. The Claws. — In giving birds the power of flight, nature would only have done half her work if she did not at the same time so modify the mechanism of their feet as to enable them to rest with security upon the lofty branches of the trees to which their wings transport them. We find, accordingly, that the con- formation of the foot is beautifully adapted to this purpose, and thus that the usual harmony and unity of design is mani- fested here, as in other parts of the animal kingdom. It has been already explained that the bone of the leg, extending upwards from the foot towards the body, is not, as might at first be supposed, the analogue of the human leg between the knee and the ankle, but the tarsus here reduced to a single bone of vastly greater proportional length than in the human foot, and having a position nearly vertical, instead of one nearly horizontal. The analogues of the toes articulated with the lower extremity of this tarsal bone are the claws, the number of which is usually four corresponding to the thumb and three fingers, or the great toe and three lesser toes. In most cases, the first is presented backwards, and the other three forwards (figs. 110, 118, 122, 154 ANIMAL PHYSICS. 124, 126). The number of phalanges or joints in these toes is different ; the thumb or backward toe having only two, and the number increasing from toe to toe, being five in the outward one. But these arrangements are subject to variation in different species, according to the exigencies of their habits and mode of life. In all cases, however, of birds which are endowed with powers of flight, the claws are so articulated as to give them such flexibility that they can grasp firmly the branch upon which they perch, 'without extraordinary effort, and cling to it with such tenacity, that even when it is swayed to and fro by the wind, the bird will hold its position with perfect security. 175. Perching. — It is further worthy of remark, that this strong hold of the bird in perching is produced without any fatiguing exertion of the muscles. The muscles which move the claws have tendons, which, passing along the tarsal bone, are connected with the extensor muscles of the upper articulation of the leg ; and when the bird perches, the weight of its body alone, acting on the flexor muscles of the claws, reacts through the tendons of these upon the ex- tensor muscles of the upper part of the leg ; so that the body is kept from sinking upon the legs, not only without any fatiguing exer- tion of the extensor muscles, but without any exertion of them what- ever. Consequently, we see the bird rest upon a movable perch with as much security when it sleeps as when it wakes. Fig. 129. E. An.) THE STOBK. 176. Standing on one leg. — In the structure of the legs of some species, such, for example, as the stork (fig. 129), a mechanical pro- vision is made by which it can rest with perfect security and stability, and without any muscular exertion what- ever, and even sleep, on one leg. The lower extremity of BIRDS. 155 the thigh hone, in such cases, presents downwards a deep cavity, into which, when the leg is extended vertically, a projection upon the upper end of the tibia enters, so as to form a stiff joint. The two parts of the leg, in this position, form a sort of vertical post, on the summit of which the body of the bird is placed, and the centre of gravity is brought over the base of sustentation formed by the line joining the extre- mities of the claws, partly by the power of the bird to incline its body upwards, and partly by the flexibility of its neck, by which the head can be thrown backwards. The bird rests in this position without any continued action of the extensor muscles ; and when it desires to resume the position suitable for walking, the articulation of the leg, thus rendered tempo- rarily stiff, is restored to its flexibility by a muscular action upon the bones thus connected in the manner above described. Fig. 131. (K. An.) THE WOODPECKER. Fig. 130. (R. An.) THE PARROT. 177. Climbers.— In general, the distribution of the four 156 ANIMAL PHYSICS. claws of perching birds is as described above, three forwards and one backwards ; but in species whose habits lead them to practise the action of climbing, such as parrots (fig. 130), toucans, and woodpeckers (fig. 131), they are differently distri- buted, the first and fourth being turned backwards (fig. 120). 178. Birds that do not fly. — That the peculiar structure of the foot just explained is designed by nature to confer upon the animal functions complementary to and concomitant with those of the wings, is proved by the fact that in species like the partridge (fig. 132), which have very imperfect powers of flight, and which do not perch at all, the number of claws are reduced to three, which have little flexibility, the fourth being rudimentary ; and in species such as the cassowary (fig. 108), and the ostrich (fig. 133), which have no powers of flight at Fig. 132. (R An.) Fig. 133. (R An.) THE SNOWY PARTRIDGE. THE AFRICAN OSTRICH. all, but, on the contrary, can run with the swiftness of a horse, the claws are reduced to two, the third being rudimentary. The same conformation is observable in the locomotive appa- ratus of the bird called the serpent-eater, which requires great swiftness of foot to pursue its prey. In the case of certain birds of prey, the claws ai-e used for prehension as well as for BIEDS. 157 support, and are accordingly constructed with a strength pro- portionate to the magnitude and weight of the objects of prey which they have to lift into the air, as in the example of eagles and vultures (figs. Ill, 112, 113, 116). 179. Waders. — Birds which live upon the banks of rivers and lakes, or the shores of the sea, and which feed upon worms and the smaller species of fish found there, holding a place intermediate be- tween land and aquatic birds, though unprovided with any apparatus to enable them to swim, have nevertheless a length of leg sufficiently great to enable them to wade in the water in search of their prey, and a length of bill sufficiently great to enable them to seize Fis- 134- (R- Au ) it. An example of THE long-legged plover, or stilt-bird. these is presented by the stilt-bird, or long-legged plover, shown in fig. 134. The class of birds to which this belongs is denominated the waders. 180. Web-footed Birds. — In fine, the class of birds which find their food floating near the surface of the deeper waters are supplied with legs, which are very imperfect instruments of locomotion on land, but perfect in their action as propellers when they float upon the surface of the water. The legs in this case are short. Powerful extensor muscles, having their origin in the upper part of the leg, act upon the claws, so as to drive them backwards, extending them in the direction of the leg, and at the same time inclining the leg more or less backwards ; and, consequently, acting upon the water like an oar or paddle. When the leg is carried forwards, previously to another stroke, flexor muscles bring together the claws, the intermediate web being collected into folds, and other flexor muscles, bending the leg upon its upper articulation, carry it forward, the claws during this motion being held together, and the web being still folded, so as to offer little resisting surface to the water. Previously to recommencing another stroke, one set 158 ANIMAL PHYSICS. of muscles separate the claws, extending the web slightly between them, after which, the extensor muscle of the legs straighten it, throwing the foot at the same time backwards. And by the constant repetition of this action Fig. 136. (R. An.) THE EIDER DUCK. with both feet, the bird is propelled forward on the water. All species of water-fowl, such as geese, ducks, swans (figs. 135 and 136), are examples of this. Fig. 135. (R. An.) THE KING DUCK. BIRDS. 159 181. Prehensile Organs of Birds. — The principal and gene- rally the only organ of prehension of birds is the beak, the larger birds of prey using occasionally for this purpose the talons. The form of the beak is as various as the qualities of the substances used as food ; and so close and invariable is this relation between the mechanical structure of the instrument of prehension and the aliment, that a practised naturalist can infer the one from the other with unerring certainty. Thus the fossil beak of an extinct bird informs us of its habits, and structure, in the same manner and by the same analogies as does the tooth of a mammifer. Tlie bill, or beak, forming tbe mouth of a bird, consists of a solid, horny substance, sharp at its edges, and variously formed at its extremities. As the mouth is never supplied with teeth, the food undergoes no process of mastication. With carnivorous birds, which have need of tearing their prey, such as hawks, eagles, and vultures (figs. Ill, 112, 113, 115, 137), the upper mandihle is very short, strong, hooked at the end, and terminated in a sharp point ; sometimes its edges are more or less den- tila ted, which renders it a more powerful arm of attack ; Fig. 137. (R. An.) and the habitudes, more or less sanguinary, of the species the head op the may be inferred as these several characters are more or falcon. less pronounced in the beak. Thus, the falcon (fig. 137) is, of all birds of prey, that whose beak is shortest, most curved, most I-'ig, 13S. (R. An.) THE GOSHAWK. completely dentilated, and, in proportion to its general bulk, most robust. ANIMAL PHYSICS. ICO 182. Goshawk. — According as species become less fierce, these characters in the structure of the beak are more subdued. Thus the goshawk (fig. 1 38), a bird of ignoble prey, has its beak curved from its base, its wing shorter than its tail, its tarsi long, and its claws curved and sharp. Its flight is rapid but low, and it pounces obliquely upon its prey, sometimes pursuing it in flight. 183. The Kite, which differs so little in its general form from the falcon, has a beak more feeble, less hooked, and not at all dentilated at the edges. Another variety, the kite of Carolina (fig. 139), called by naturalists Elarnus, has the same characters, but is distinguished by the legs being half-covered with feathers. We find accordingly, as indicated by the beak, that these species are much less fierce than the falcon. Fig. 139. (R. An.) THE KITE OF CAROLINA. The vulture (fig. Ill), whose beak, though more hooked than that ot the falcon, is longer and consequently weaker, never attacks living prey, feeding exclusively on carrion. 184. Sea-Birds, which feed on fish too large to be swallowed at a mouthful, are furnished with a large beak, hooked at the end (fig. 125). But this instrument is much longer, and there- BIRDS. 161 fore less powerful, though sufficiently so relatively to their prey. When piscivorous birds feed on such fishes and reptiles as are small enough to be seized and easily swallowed, the beak is straight, still greater in length, and resembling a pair of long pincers, of which those of the martin pecker (fig. 140) and stork (fig. 129) are examples. 185. Insectivorous Birds, such for example as the bee-eater (fig. 141), have slender and very long beaks, either straight or very slightly hooked, except when they catch their prey in Fig. 140. (R. An.) THE MARTIN PECKER. Fig. 141. (R. All.) THE BEE-EATEPw. flight, as do the swallow and the goatsucker (fig. 142), for ex- ample, in which the bill is short, broad, and deeply cut, so as to enable them to present a large mouth to receive them prey. Fig. 142. (K. Au. THE GOATSUCKER. Fig. 143. (11. Au.) THE SPARROW. 186. Granivorous Birds, on the contrary, such as the spar- row (fig. 143), have a short thick bill, convex above, or conical, and in general straight, the upper mandible not projecting over the lower. 162 ANIMAL PHYSICS, 187. Pelican. — A singular modification of this organ of pre- hension is presented in the case of the pelican (fig. 144), v,hich Fig. 144. (R. An.) THE PELICAN. Fig. 145. (R. An.) the horn bill. , membranous receptacle, consisting of a cutaneous pocket BIRDS. 163 or bag, attached to its lower mandible, in which it collects its prey, which it swallows afterwards at leisure. 188. Hornbill. — In fine, appendages are found occasionally upon the organ of prehension of certain birds ; such, for example, as the hornbill (fig. 145), the use of which has been hitherto undiscovered. 189. Fossil Birds, like mammifers, are only* found in the tertiary group, and there increase gradually in number in ascend- ing from the inferior to the superior strata. Owing to the orders in which birds have been classed being determined by Fig. 146. (R. An.) FOOT-rRINTS, AND MARKS OF RAIN-DROPS. the bills and claws, which are scarcely ever preserved in the fossil state, the classification of fossil birds has presented As in the cases of mammifers, some traces have been found iu the lower groups, consisting, however, of foot-prints. M 2 164 ANIMAL PHYSICS more difficulty than lias been encountered in mammifers, the orders and genera of which are determined by the bones. Nevertheless, the inquiry has been facilitated by the foot-prints which, in many cases, have been found in rocks which received them in the soft state, and were subsequently solidified without losing them. In fig. 146, the imprint of the three fore phalanges are shown, accompanied by the curious incidental im ressions of rain-drops which happened to fall at the Fig. 147. (R. An.) FOSSIL BIRD FOUND IN THIS GYPSUM OF MONTMARTRE, PARIS. same moment. In fig. 147 is a mould of the chief parts of a skeleton, including the beak. The gradual increase of bii'ds with the succession of geological periods is shown by the fact, that while only 11 genera have been found in the lower strata of the tertiary rocks, 29 have been found in the upper strata, the existing genera amounting to about 300, which include above 5000 species. The foot-prints which have been observed generally consist of three front phalanges, and sometimes of one posterior. The succession of impressions shows the animal to be biped. The magnitude of the feet and the length of the steps, sometimes enormous, prove the existence of birds in these geological epochs, of vastly greater magnitude than any of the existing specimens of the class. In one case, the foot-prints are 1 5 inches in length and 10 inches in breadth, without counting the posterior phalange, which measured two inches. The length of the steps were from four to five feet, while those of the ostrich are seldom so much as a foot. REPTILES. 1 90. Form and Structure. — The animals assigned by natu« REPTILES. 165 ralists to this class differ one from another extremely in their Fig. 14S. (R. An.) THE GREEN LIZARD. form and structure, as may be imagined when it is stated that they comprise species so very unlike as tortoises and serpents. The characters by which the class is zoologically distinguished have refer- ence to their circulating apparatus and to the physical condition of their blood, as will be explained in a suc- ceeding chapter. The head of the reptile is small, and the body generally, but not always, long in proportion to its diameter. Some species, such as tortoises (fig. 149), lizards (fig. 148), and frogs, have four legs, the feet of which are formed for moving either on the ground or through the water. In some species the legs are merely rudimentary (fig. 150) ; and in others, such as serpents (fig. 151), they are altogether wanting. The legs in such species as possess them are always so short, Fig .149. (R. An.) THE GREEK 'JOR'IOISE. Fig. 150. (R. An.) CHALEI8. and their action so nearly lateral, that they supply very imper- 166 ANIMAL PHYSICS. feet instruments of locomotion, so that the body trails on the Fig. 151. (R. An.) RAJA ASPIC. ground ; whence the class takes its name, from the Latin word repo, signifying I creep. 191. Serpents. — In these species, such as the varieties of ser- pents where the legs altogether disappear, locomotion is effected by the contractile force of the muscles alternately drawing up and extending the body, combined with the adhesion of the tegu- mentary covering with the ground. The animal attaching to the ground a point near its head contracts its bod}', or even bends it into an arch, bringing forward the hinder part, some point of which it then attaches to the ground, liberating at the same time the fore part. The posterior point of attach- ment then becoming a fixed point, the animal throws forward its length by the action of its extensor muscles, after which it again attaches a point in the foremost part of its body to the ground, and repeats the same process. The two points thus alternately taken as points of re-action may be close to each other, and other pairs of similar points may be at the same time in operation at other parts of the body. By a series of such points of re-action an animal of this class can give itself a progressive motion by the alternate contraction and extension of its body without arching or elevating any part of it from the ground. KEPTILES. 167 1 92. Poisonous Serpents. — Nature has provided several species of serpents — such as the viper, asp, and rattlesnake — with an apparatus by which a specific poison is secreted, and poured into the wound at the moment the bite is accomplished. This poison is in general so deadly, that it strikes the animals on which these creatures prey with almost instantaneous death. The venomous fluid is secreted in glands placed in the upper jaw, from which ducts are carried to a pair of teeth of peculiar form and structure, specially appropriated to inflict the poison- ous bite. The ducts enter the roots of the teeth at the embouchures of perforations which are continued through the teeth, and terminate with openings near the points. The venomous glands are placed in immediate con- tact wdth the temporal muscles, so that the same act which moves the jaws in inflicting the bite, compresses the gland, and expels the poison through the duct and the dental canal, so that it is poured into the wound at the moment the tooth pierces the flesh. It is remarkable that this poison, deadly as it is when applied in the manner in which the animal imparts it, is absolutely innocuous if taken into the mouth and stomach. When inflicted by the bite, it is mixed with the blood, and carried by its current through the circulation, where alone it is destructive ; the effect being to retard the circulation, to destroy the coagulability of the blood, and to accelerate the gangrene and mortification in the wounded part. The mouth of the rattle-snake (fig. 153), with its venomous apparatus, is represented in fig. 152, where v is the venomous gland, the excreting duct of which terminates in the large moveable tooth c. The levator muscles, m, of the jaw invest and compress the gland s, the salivary glands are disposed along the edges of the jaws, and below n, the nostril, appears a second similar opening, by which these and some other serpents are distinguished. n v m 193. Remedies. — The most effectual remedy against the consequences of such poisonous bites, is immediately to sus- pend the local circulation by compressing the veins above 163 ANIMAL PHYSICS. the point attacked that is to say, between the point and Peart— so as to prevent the flow of the poisoned blood into the system, and at the same time to apply suction to the wound either by the mouth, or, still better, by a cupping instrument. That sucking by the mouth may be applied with impunity is proved by the fact already mentioned, that the poison, though destructive when it enters the circulation, is altogether innocuous when it passes through the alimentary canal. Cauterisation, either with red-hot iron or with any strong chemical agent, is also effectual, but such remedies as ammonia and arsenic are of doubtful efficacy. The South American Indians, who are exposed to the frequent attacks of these reptiles, have great faith in the remedial virtue of a plant of those countries called guaco, which they affirm to be not only efficacious as a cure for the bite of the venomous serpents, but that inoculation by its juice will repel the reptile, and prevent it from attacking the individual thus prepared. It would appeal that Humboldt himself did not altogether discredit this popular remedy, and considers it possible, if not probable, that Fig. 153. THE RATTLE-SXAKE. the plant may impart an odour to the person inoculated such as to repel the reptile. REPTILES. 169 194. The Skeleton of Reptiles is not so uniformly con- structed as that of mammifers. With the exception of the cere- bro-spinal column, every part of it is absent in one species or another. "When present, however, the various bones consti- tuting it are always analogous to those of birds and mam- mifers. The head is small, and the face elongated. Like that of birds, the lower jaw consists of several parts, and is articulated to the temporal bone by one, and sometimes two, intermediate pieces. It is owing to this mechanism that serpents are enabled to open their jaws so widely as to swallow their prey whole, that prey being often larger than their entire body. The skeleton of the head of the rattlesnake (fig. 153) is shown in fig. 154. A short moveable bone, called the mastoid, n, is articulated to the skull, c, and connected with it by ligaments and muscles. To this another bone b is jointed, and to the lower extremity of the latter the branch of the lower jaw, a, is articulated. The two branches of the lower jaw are not connected. Thus, the utmost freedom of motion is given to this part ; and owing to the length of the inter- mediate bone b, a great capa- city is obtained at the in- terior part of the gullet as well as at the front of the mouth. The branches of the upper jaw are only attached to the intermaxillary bone by ligaments which allow them a certain play, in which the SKELET0N 0F THE HEAD 0F A Rattle-snake. bones of the palate, o o, par- ticipate. This structure is in remarkable accordance with the habits of the animal, which engulphs large masses of aliment at a single meal with- out mastication, during the long digestion of which it remains in a state approaching to torpidity. The jaws are furnished with sharp, hooked teeth, which are however adapted, not at all for mastication, but merely for the seizure and retention of its prey previous to deglutition. An increased power is given to the mouth by a certain limited mobility which is allowed to the upper jaw. 195. The Skull of reptiles, which is generally fixed, or nearly so, is connected with the vertebral column by a single condyle having several facets. 196. Trunk — In lizards, crocodiles, and reptiles similarly formed, the bones of the trunk present but few anomalies. The ribs are more numerous than in mammifers and birds, being G G A Fig. 154. 170 ANIMAL PHYSICS. continued over the lumbar portion of the vertebral column as well as the dorsal. In serpents, the sternum or breast-bone is absent, and all the ribs consequently have the mechanical character of the false ribs in mammifers. Their number is sometimes prodigious. In the adder, for example, there are above three hundred pairs, the mobility of which, combined with that of the vertebrae, has an important share in facilitating the movements of that animal. 197. Skeleton of the Tortoise. — One of the most remark- able examples of the expedients by which nature attains her pur- poses without departing from the general plan which she appears to have prescribed to herself in the construction of the solid CcV Coras ciV Rb Rb Re Ti Fi Fo Fig. 155. (R. An.) SKELETOX OF THE TORTOISE. frame-work of vertebrate animals, is presented in the case of the tortoise, where all tlio principal parts of the skeleton of the REPTILES. 171 superior animals can be recognised, though signally changed in their form and proportions. The tortoise, as is well known, is enclosed between two shells, one of which, the carapace, covers its back, and the other, the plastron or breast-plate, its belly. These two are united at the sides, leaving an opening before and behind, through which the animal can put out its head and fore-legs in front, and its posterior members behind. Now the carapace or shell, which covers the back, is the homologue of the vertebral column and ribs. The ribs are flattened out, and cemented together edge to edge, so as to form a continuous shell. The shell which covers the belly is the analogue of the sternum or breast-bone, and the cartilaginous parts of the ribs, in the superior species. The former is flattened out ; and the latter, like the ribs, are also flattened, cemented edge to edge, and hardened to the consis- tency of a shell. Compared with the sternum of the superior animals, it is extended longitudinally as well as laterally. This bony box is invested in a tegumentary covering, on the external surface of which, however, no muscles are inserted ; all those attached to it being confined to the internal surface. The scapula, instead of being outside, is inside the thorax. The pelvic bones are in like manner within the abdomen. The skeleton of the tortoise, divested of the shell which covers its belly, and seen from below, is shown in fig. 155, the several parts being indicated by the same letters as in figs. 8G and 92. 198. Lizards. — In other reptiles of this class the structure of Fig. 156. (R. An.) THE GECKO. the shoulder bears a closer resemblance to that of birds. The legs 172 ANIMAL PHYSICS. are sometimes truncated, and, with the land-tortoises (fig. 149), serving merely to push the animal forward. Sometimes they are furnished with nails or claws, by which the animal can hook itself to the inequalities of the ground, and cling to objects in a vertical position as lizards do. Sometimes, as in the case of the wall-lizard, called the Gecko (fig. 15G), they have a sort of suckers to their feet, by which they can run up the smoothest walls. In reptiles which chiefly inhabit the water, such as the sea- Fig. 157. (R. An.) TUB SEA-TOKTOISE. tortoise, the extremities are formed into a sort of paddle unsuitable for moving on the ground, but adapted for swimming. 199. The Dragon. — Among the existing reptiles, there is Fi«. 15S. (R. All. one called the dragon (fig. 158), which combines the powers of REPTILES. 173- crawling and flying. Resembling in its general form a lizard, or saurian, it is furnished with a web projecting from its sides, like those which form the wings of a bat, but which, instead of being moved by the members, is altogether independent of them, and is sustained by the ribs. The animal does not work this web as birds do their wings, but merely uses it as a para- chute to moderate its fall, as it leaps from branch to branch. These singular reptiles, which inhabit India, realise the poetical fiction of flying dragons ; with this difference, however, that instead of being formidable by their magnitude and voracity, they are small, and prey only upon insects. 200. Extinct Reptiles. — One of the uses of thus tracing the connection between the mechanical structure of the skeleton and the powers of locomotion conferred upon the animal is, that the generalisations which result from such analogies supply means of discovering the functions and habits- of animals with whose skeletons alone, or even with certain parts of them only, we are acquainted. Such reasoning has conducted those who have devoted then labours to geological researches to the most surprising discoveries. The skeletons of animals which inhabited the earth and had disappeared from it thousands of ages before the appearance of man upon it, being found imbedded in rocks, have supplied all the data necessary to discover their modes of life, their habits, and the sort of food upon which they subsisted. In some cases, a part only of the bones have been found, which nevertheless the general analogies of structure have rendered sufficient to enable anato- mists to infer all those that are deficient ; nay, such is the perfection to which comparative anatomy and physiology have- been carried in these our times, that a single bone has been sometimes found sufficient to enable anatomists to trace tho whole skeleton. 201. Fossil Saurians.— Among the monstrous animals which inhabited the globe at these remote geological periods, whose remains are now found usually in a fragmentary state imbedded in the crust of the earth, the most remarkable are a class of amphibious or aquatic monsters, to which geologists have given the generic name of Sav/ria/ns, from the Greek word aavpos (sauros), meaning a lizard. Tho skeletons, though often incomplete, nevertheless enable us, by anatomical analogies, to determine the form, dimensions, habits, and modes of life of 174 ANIMAL PHYSICS. these prodigious reptiles. Numerous species have been dis- covered agreeing in their general resemblance to the crocodile and the lizard ; their bodies were covered with scales ; they had four members, the phalanges of which were furnished with claws. Then- mouths were armed with formidable teeth, and all had a tail of greater or less length. Their general mag- nitudes far exceeded that of the largest of this class of reptiles at present existing on the earth. Geologists have distinguished these extinct animals into many species, which they have named either from their magnitudes or their resemblance to known animals. Thus one species, from its vast magnitude, is called the Megar losaurus, from the Greek word yeyaXos (megalos), great ; another is called the Ichthyosaurus, from its resemblance to a fish, the Greek word l\Qvs (ichthus) signifying a fish ; another is called Plesiosaurus, from the Greek word TrXrjcrins (plesios), ‘ next to,’ from its close resemblance to the crocodile. 202. Megalosaurus. — The most remarkable of the saurians is the Megalosaurus, discovered by Dr. Buckland imbedded in the strata of oolitic slate at Stonesfield, near Oxford. The remains found show that these animals had a form partaking of the structure of the crocodile and monitor, and a length of from forty to fifty feet. The femur and tibia, or thigh and leg bones, which have been found, measure three feet each, and the metatarsal or instep, thirteen inches. The habits of this class of animals and the nature of their food have been generally inferred from the structure of their teeth, which Dr. Buckland describes as consisting of a combination of mechanical contri- vances involving at once the principles of the knife, the sabre, and the saw. When first protruded from the gum, the apex of each tooth presented a double-cutting edge of serrated enamel. In this first stage, its position and line of action were nearly vertical ; its form resembling the two-edged point of a sabre, cutting equally on each side. As the tooth advanced in growth, it became curved backwards in the form of a pruning- knife, and the edge of serrated enamel was continued down- wards to the base of the inner and cutting side of the tooth, whilst on the outer side a similar edge descended but to a short distance from the point, and the convex portion of the tooth became blunt and thick, as the back of a knife is made thick for the purpose of producing strength. FOSSIL REPTILES. 175 In a tooth thus formed for cutting along its concave edge each movement of the jaws combined the powers of the knife and saw, whilst the apex, in making the first incision, acted like the two-edged sabre ; the backward curvature of the full- grown teeth enabled them to retain, like barbs, the prey which they had penetrated.* 203. Fossil Reptiles, unlike the superior vertebrates, are found in all the rocks, from those which lie over the Devonian and Silurian groups, to the upper tertiary, which are in imme- diate juxtaposition with the superficial strata. The remains discovered in all these groups of rocks are not only numerous, but those of individual specimens are found in a state of greater integrity and perfection than those of mammifers or birds, the deficient parts of which are reproduced by osteological analo- gies. Reptile skeletons, on the contrary, are sometimes found complete, with all their parts in their proper relative position, and even the vestiges of their connecting tendons and of the muscles which gave them motion have been traced. When the entire skeleton is not thus found, the heads, with the vertebral columns, and the component vertebrae are present. Such remains are recovered in great numbers in all the strata from the lowest of the secondaiy group to the uppermost of the tertiary. The Chelonians, + or land and sea tortoises, are diffused through all the strata above the trias group. The Saurians, or crocodile and lizard tribes, are diffused through all the strata above the carboniferous limestone. Of these, numerous genera existed in the remote geological epochs which have since become extinct, and which have even disappeared in the superior and more modern strata. Thus there are not less than twelve genera of Saurians found in the trias group, and ten in the J urassic or Oolitic group, which have disappeared from all the more recent formations. 204. The Ichthyosaurus, (fig. 159) or fish-lizard, is an example of an extinct animal of this tribe, which has the muzzle and general aspect of a porpoise, the head of a lizard, the teeth of a crocodile, the vertebra; of a fish, the sternum or breast-bone of an ornitborliynchus, and the fins of a whale. The enormous magnitude of the eye-balls was one of the pecu- liarities of this genus. The cavities in which they were lodged, in one of the species, measured not less than fifteen inches in diameter. The eye-ball was protected by a ring of bony plates resembling those still seen in birds, tortoises, and certain saurians, the use of which was doubtless to push forward or draw backward the cornea, so as to increase or dimi- ‘ Buckland, Bridgewater Treatise, vol. i. p. 238. + XfAuyt] (cheloix!), a tortoise. 176 ANIMAL PHYSICS. nisli its radius, and thus vary the range of distinct vision. This, combined with the great power of the fins or propellers, must have conferred upon the reptile great promptitude in perceiving and seizing its prev. These reptiles were essentially aquatic, and the form of their teeth proves them to have been carnivorous. Their coprolites, or fossilised Fig. 159. THE ICHTHYOSAURUS. excrements, show that their intestine was spirally arranged, like that of certain fishes. This genus was most prevalent during the period of the formation of the lias rocks. 205. The Plesiosaurus, a reptile closely allied to the lizard tribe, (fig. 160,) had the head of a lizard, a neck of prodigious length, resembling the body of a serpent, the tail of a quadruped, the ribs of a chameleon, and the fins of a whale. This monster of a remote epoch of the globe has been compared to a serpent supplied with the shell of a tortoise. While reptiles in general have but eight cervical vertebrae, this had thirty-three. I Mr. Conybeare, to whom its discovery is due, concludes that it was a monster of the deep ; that, by the enormous length of its neck, it was j enabled to dart upon its prey without moving from its place. These reptiles I are first seen in the trias group ; they were most numerous at the epoch of the deposition of the lias strata of the Jurassic group, and disappeared at that of the formation of the upper strata of the same group. 206. The Cetiosaurus,* or whale-lizard, the discovery of which is due I to Professor Owen, is characterised by spongy bones, and the absence of the I medullary canal in the long bones. These reptiles had a magnitude equal to that of the largest whales. They first appear in the upper strata of I the Jurassic group, but are more numerous in the lower strata of the | * Cetus, a whale. FOSSIL REPTILES. 177 cretaceous. The only existing genus is the crocodile. The jaw of an alli- Fig. 101. gator of this family, found in the tertiary strata of the Isle of Wight, is shown in fig. 161. 207. Fterodactyle.— The epoch of the saurians was also signalised by its flying reptiles, on a scale, however, vastly greater than the dragon of the zoologists. Among them was a Fig. 162. THE PTEKODACTYLE. species of gigantic bat, which has received the name of Ptero- dactyle, from two Greek words, nrtpov (pteron), a wing, and baKTv\of (daktulos), a finger ; inasmuch as one of the fingers, being of extraordinary length, is supposed to have had an extensive web attached to it, which it extended, in flight, hi 178 ANIMAL PHYSICS. the same manner as the sail of a ship is extended upon its yard. The skeleton of this animal, with the outline of its wing, is shown in fig. 162. AMPHIBIA. 208. This class, as their name implies, is capable, during a certain portion of their existence, of living indifferently on the land or in the water, and therefore hold an intermediate place between reptiles and fishes. During the first period of life they generally dwell altogether in the water, to which alone their structure is then adapted ; their respiration being effected, like that of fishes, by means of gills, which, being washed by the water, take from it the air fixed in it. As the animal grows, the lungs become developed, and in most species the gills gradually disappear. The legs, which are generally absent in the young, are also put forth, and the animal acquires at once powers, though very limited ones, of aerial respiration and locomotion. The members, constructed in accordance with their habits, are adapted, however, chiefly to aquatic propulsion. As in the cases already noticed, the general plan of the skeleton is conformable to that of man and the superior classes of vertebrated animals, the special functions being obtained by mere modifications of the parts. The parts corresponding to the arm and fore-arm being very short, allow the muscles to act upon them with great effect. Great breadth, however, is given to the hand, and length to the fingers, which being enclosed in a strong and tough membrane, convert the hand into a sort of paddle. The structure and number of the bones correspond to those of the human hand ; and although its external form differs much from that member, a striking similitude will be apparent in the skeleton. 209. The Seal.*— An example of this is presented in the Fig. 103. (R. Au.) THE SEAL. case of tho seal, fig. 163, the skeleton of which is shown • According to some naturalists, Seals, Otters, &c., arc not strictly amphibious. AMPHIBIA — FISHES. 179 in fig. 164, -where the parts corresponding to those of the dv cv Fig. 164. (R. An.) SKELETON OF THE SEAL. human skeleton and that of other vertebrated animals are indicated. The phalanges of this class differ much in certain species from those of land animals, the number being often very con- siderable ; and they are sometimes replaced by a multitude of small bony rods connected together by a membrane resembling the fins of fishes. FISHES. 210. The animals which inhabit exclusively the waters have generally forms peculiarly adapted to move through a liquid. Inequalities such as those which characterise the form of land animals would prove an impediment to their motion ; the head is therefore connected with the body without the intervention of a neck ; nor are there any extensive projecting members to the motion of which the fluid would offer any resistance. The longitudinal section of the body presents a close approximation to the form determined by mathematicians as that of the solid j of least resistance — tapering towards the head and tail, and gradually enlarging towards the middle. The skin is smooth but scaly, offering little resistance to friction directed from the head towards the tail, but a considerable resistance in the other direction, owing to the position of the scales. The anterior ) and posterior members are replaced by fins, tho phalanges n 2 180 ANIMAL PHYSICS. consisting of a series of thin bony rods, not articulated, having a web extended between them. 211. Fins. — Some few species are deprived of fins, but most species are supplied with these appendages, which are the analogues of the four members of the superior classes. The anterior fins, corresponding to the human arms, the fore legs of quadrupeds, and the wings of birds, are placed at each side of the trunk immediately behind the head, and are deno- minated, from their position, pectorals. Those which corre- spond to the human legs, the hind legs of quadrupeds, and the legs of birds, are closer together, and generally placed on either side of the lower part of the body, being found in different species in various positions between the head and the tail ; these are called ventral fins. Other organs of the same form, having no analogues in the superior animals, are found in certain species placed in the median plane on the back or belly, and, unlike the other fins, they exist singly, and not in pairs. These are variously denominated, according to their position, dorsal, anal, and caudal fins. 212. Gills. — On either side of the head are openings corre- sponding with the position of the ears, in which are deposited the gills, which play the part of the lungs, being endowed with the functions of aquatic respiration. The water entering in front, and escaping through these lateral openings, washes the foliated structure of the gills, which absorb the air suspended in it. 213. Scales. — The tegumentary covering of the body consists generally of scales of various form and structure, but disposed most commonly one upon the other like the scales of armour, and so lying that, as the animal advances through the water, the pressure of the fluid upon them shall tend to keep them in close contact, and not to enter between scale and scale. These scales are remarkable for their brilliant colours, having gene- rally a metallic lustre, and often reflecting, like mother-o’- pearl or shot-silk, the richest hues, such as greens, blues, reds, purples, and so on. 214. Skeleton.— The skeleton of fishes is generally, but not always, osseous ; certain species being exclusively cartilaginous. FISHES. 181 The composition of the bones differs from those of mammifers by the absence of gelatine. Fig. 165. (R. An.) The form of the head resembles that of the prow of a vessel. Nature, therefore, appears to have conferred upon them that peculiar configuration which is most favourable to their move- ment in the element in which they dwell. 215. Organs of Natation. — These animals in general swim with great agility. It is said that the salmon, for example, can move through the water at the rate of from twenty to twenty- five miles an hour. They propel themselves through the water by striking that fluid laterally by the alternate inflexion of their trunk and tail, the fins in which the tail terminates acting on the water like an oar in sculling a boat. To confer upon the animal sufficient strength for this important action, it is furnished 'with powerful muscles by which the lateral flexion of the vertebral column is produced alternately on one side and the other. These muscles are developed to such an extent that they alone form the chief part of the volume of the body. The caudal, dorsal, and ventral fins co-operate in the propulsion to a certain extent, but the lateral ones have no other influence than that of directing the course of the animal, and of main- taining it in equilibrium. 216. Air-bladders There is a particular organ in fishes called the air-bladder, which has an important share in the powers of aquatic locomotion which this class of animals enjoy. Placed within the abdomen under the dorsal part of the spiual column, it 182 ANIMAL PHYSICS. communicates in many species with the oesophagus or with the stomach by a canal through which the air contained in it can escape, but in general the air does not appear to enter by this route. It is in most cases a secretion of the glandular sides of the air-vessel itself, which is sometimes completely closed. By the play of the ribs this elastic bladder is more or less com- pressed, and accordingly gives greater or less buoyancy to the body of the fish, which is enabled by this means at will to rise towards the surface, sink into deeper water, or continue at the same level. That such is its especial purpose is demonstrated by the fact that in rays, soles, turbots, and eels, which remain either at the bottom or buried in mud, this organ is either very small or altogether absent. It is sometimes membranous, receiving numerous capillaries like a lung, which it repre- sents in a rudimentary state, and the functions of which it may possibly to a certain degree be endowed with. 21 T. Flying-fish. — A few species of fishes possess locomotive organs, which enable them to launch themselves out of the water and sustain themselves for a few moments in the air. The flying-fish (fig. 166) presents an example of this. There are also some which, by creeping and jumping, are enabled to move upon the Fig. 166. (R. An.) ground, and some are even cited which climb up trees ; but these examples are extremely rare, and some of them of questionable authenticity. 218. Sucking-fish. — Among the prehensile and motor organs of fishes, those by which they are enabled to attach themselves with extraordinary tenacity to external objects, ought not to be omitted. The Remora (fig. 167), or sucking-fish, which prevails in the Mediterra- Fig 167. nean and some other seas, presents a remarkable example of this class. The organ in question consists of a flat oblong disc composed of cartilagi- FISHES. 183 nous movable plates, directed obliquely backwards, established over the head. The surface of this disc is represented in fig. 168. This species has been long celebrated on the shores of the Mediterranean, and its history is overlaid with fable. Thus it is pretended that it feeds by a species of suction exercised by the disc just described ; and the mariners of the coasts ascribe to it the power of suddenly stop- ping a vessel in its most rapid course. It appears, on more authentic grounds, that this sucking instrument is used for the seizure of its prey, and fishermen in the seas on the coast of Cafl'raria use it to catch other Fig- 168. fish. For this purpose, after having tied a line to its tail, they launch it into the water, and when it attaches itself to its prey they draw it out. 219. Mode of Propagation. — Fislies are propagated by means of eggs, the number of which produced at a single laying is often immense, amounting sometimes to several hundred thousand. In general the eggs are enveloped in a mucilaginous covering, and are only fecundated after being laid. Some species, how- ever, are viviparous ; a word by which naturalists express a mode of generation in which the young has been already extri- cated from the egg at the moment of birth : but whatever be the manner in which the young of fishes come into fife, they are invariably abandoned from the moment of their birth, and a very large proportion of them perish immediately after- wards. It is to the simultaneous development of an enormous number of eggs deposited in one place, and to the instinct which impels the animals thus produced to keep together, that is to be attributed the assemblages of the immense legions of certain sorts of fish to which the fishermen have given the name of banks or shoals. Such assemblages, however, cannot properly be called societies. Between the individuals which compose them there is no interchange of services ; the same physical wants probably keep them in the same locality, or impel them to the same change of place ; and if the whole assemblage be observed to follow some among their number as their guide, it is merely an effect of that tendency to imitation which always attends the first glimmerings of intelligence. 220. Migrations. — These animals, thus assembled in troops, often make long voyages ; sometimes to gain the deep, sometimes to ascend the embouchures of rivers, or generally to change their 184 ANIMAL PHYSICS. quarters. Certain species, however, lead a sedentary life, remaining always in their original locality. Others, on the contrary, are always wandering, and a great number make periodic voyages of considerable length. In the spawning season they usually approach the coasts, or enter the mouths of rivers ; and for this purpose sometimes make an extremely long voyage. At certain seasons, vast troops of these migratory fishes arrive regularly in the same waters ; and it Ls generally believed that certain species migrate periodically from the north towards the south, and return from the south towards the north, following always a determinate route. Some naturalists, however, doubt this, and consider it more probable that the periodical change of place is limited from deep to shallow water, and vice versa. 221. The Herring.— Among these migrating fishes, the species which is by far the most important in its relation to the industry of the fisheries is the herring. This fish inhabits the northern seas, and visits annually, in countless shoals, the coasts of the Old and New Continents as far south as the forty- fifth degree of latitude. Some naturalists think that these myriads retire periodically into those depths of the Polar Seas which are below the stratum of constant temperature, and therefore defended from the rigour of the surface. Issuing from this common rendezvous, they depart at the appointed season in a prodigious column, which soon, however, sub- divides, sending oft’ large divisions and numerous detach- ments to all the continental coasts, and more especially to all the straits and channels within the limit of latitude above mentioned. It appears to he ascertained that the spawning regions of the herrings are near the coasts frequented by the fish ; and it is supposed that the young, immediately on coming to life, withdraw into deeper waters, and direct their course to the north, where they find in much greater abundance the animalcules and small Crustacea which constitute their proper food. In spring, new wants bring them back to the shores, where they seek more shallow and tepid waters. At that season they appear in countless numbers, descending southwards. 222. Periodical Voyages.— In the months of April and May they begin to make their appearance around the Shetland Isles ; FISHES. 185 and towards the end of July they arrive in incalculable numbers, forming vast shoals, which extend over the surface of the sea for several leagues continuously in length and breadth. The vast multitude of these creatures may be imagined, when it is stated that such extensive shoals often have a thickness of several hundred feet. According to Pennant, the grand winter rendezvous of the herring is within the Arctic circle ; there they continue for many months, in order to recruit themselves after the fatigues of spawning ; the seas within that limit swarming with insect food in a far greater degree than those of our warmer latitudes. This mighty army begins to put itself in motion in spring, and appears off the Shetland Isles in April and May. These, however, are only the forerunners of the grand shoal, which comes in June ; and their appearance is marked by certain signs, such as the numbers of birds, like gannets and others, which follow to prey on them ; but when the main body approaches, its breadth and depth is such as to alter the appearance of the very ocean. It is divided into distinct columns of five or six miles in length and three or four in breadth, and they drive the water before them with a kind of rippling. Sometimes they sink for the space of ten or fifteen minutes, and then rise again, and in fine weather reflect a variety of splendid colours, so that the surface of the sea resembles a field of precious gems. The first check this army meets in its march southwards is from the Shetland Isles, which divide it into two parts : one wing takes to the eastern, the other to the western shores of Great Britain, and every bay and creek is filled with their numbers ; the former proceed towards Yarmouth, the great and ancient mart of herrings. They then pass through the British Channel, and after that in a manner disappear. Those which proceed towards the west, after passing the Hebrides, where the great fishery station is established, proceed to the uox-th of Ireland, where they meet with a second interruption, and are obliged to make a second division ; the one takes to the western side, and is scarcely perceived, being soon lost in the immensity of the Atlantic ; but the other, that passes into the Irish Sea, rejoices and feeds the inhabitants of most of the coasts that border on it. These brigades, as we may call them, which are thus separated from the greater columns, are often capricious in their motions, and do not show an invariable attachment to their haunts. 186 ANIMAL PHYSICS. This instinct of migration was given to the herrings that they might deposit their spawn in warmer seas, that would mature and vivify it more assuredly than those of the frozen zone. It is not from deficiency in food that they set them- selves in motion; for they come to us full of fat, and on their return are almost universally observed to be lean and miserable. What their food is near the Pole we are not yet informed ; but in our seas they feed much on the oniscut marinus, a crustaceous insect, and sometimes on their own fry. They are full of roe in the end of June, and continue in perfection till the beginning of winter, when they deposit then- spawn. The young herrings begin to approach the shores in July and August, and they are then from half an inch to two inches long. Though we have no certain authority for it, yet, as very few young herrings are found in our seas during winter, it seems most certain that they must return to their parental haunts beneath the ice. Some of the old herrings continue on our coast the whole jmar. 223. Herring Fishery. — Towards the middle of the seven- teenth century, the Dutch employed in the herring fishery not fewer than two thousand vessels, and the industry gave employ- ment to nearly a million of persons. Although the importance of this branch of industry is not so great as it has been, it still gives employment to a large proportion of the coast popu- lation of the United Kingdom and the northern parts of Europe. The operation of the herring fishing is conducted usually with nets from five to six hundred fathoms in length, the lower edge of which is sunk by weights, and the upper edge floated by means of empty barrels. The meshes of the net are just large enough to allow a herring of ordinary size to thrust through them the head and gills, but not to let the pectoral fins pass. The fish, endeavouring to extricate itself, only becomes more entangled ; the position of its gills ■directed backwards preventing it from withdrawing, and the magnitude of its abdomen and projecting pectoral fins pre- venting it from advancing. It remains therefore a prisoner, until the nets are taken up. The number of fish taken in this way is sometimes so great, that the net is filled with them, and occasionally breaks under then- weight. Alien the place of the fishery is very distant from the port, I FISHES. 187 -the fish is salted on board, thus affording employment to the fishermen during their voyage. 224. Dulness of the Senses. — The tactile and other senses of fishes are extremely limited, and this class of animals seems to pass its life exclusively in seeking subsistence and flying from its enemies. It exhibits no remarkable instincts ; its brain is but slightly developed, and its organs of sense very imperfect. 225. Electric Fishes. — Among the offensive and defensive organs of fishes, those by which certain species are enabled to develop voltaic electricity, and to inflict an electric shock on their enemies or their prey, merit especial notice. Several species are endowed with this power, .and it is very remarkable that the electric organ differs altogether in its structure in one compared with another. 226. Gymnotus Eleetricus. — One of the species which possess this curious physical power is the Gymnotus eleetricus, or electric eel, fig. 169. This species, which inhabits Southern America, closely resembles common cels, wanting, however, the fins at the end of the tail, and no scales being visible upon its skin, which is covered with a glutinous matter. Its length is from six to seven feet, and it is commonly met with in the streams and ponds which are found in various places in the immense plains which overspread the valleys of the Cordil- leras, the banks of the Oronoco, &c. The electric shocks which the animal is enabled to give at will have an intensity sufficient to paralyse not only men but horses. It uses this organ accordingly, not only to defend itself from the attacks of its enemies, but to kill at a distance the fishes on which it feeds, the water being a suffi- cient conductor of electricity to trans- mit the shock. Its first discharges are generally weak ; but when the animal is irritated and roused, they become stronger, and at length acquire a terrible intensity. When the animal has communicated a certain number of these shocks, it becomes exhausted, and is forced to desist, and it is not until after the lapse of a certain interval that it is enabled to recommence. It would appear as though the electric organ, like the scientific machine, when once completely discharged, requires a continued action of the exciting power, which in this case is a vital function of the animal, to recharge it. 188 ANIMAL PHYSICS. 227. Manner of capturing them.— The natives of the countrie- wluch the animal inhabits avail themselves of this temporary suspension of its offensive power to capture it. Troops of wild horses are dri the reservoir in which the creature is known to prevail ; immediately the horses are fiercely attacked, receiving a rapid succession of intense electric shocks, by which they are more or less stunned and paralysed, and not infrequently killed ; but the assault has the effect of exhausting the electric eels, and rendering them comparatively inoffensive, so that they are easilv captured, either by the net or harpoon. 22S. Electric Organs. — The apparatus by which the gymnotus pro- duces these electric shocks is extended along the entire length of the back to tbe tail, and consists of four longitudinal masses composed of a great number of membranous folds, connected by an infinite number of smaller membranes placed transversely to them. Tbe small prismatic cells formed by tbe combination of these membranes are filled with gelatinous matter, and tbe whole apparatus is supplied with large nerves. 229. Fig. 1T0. THE COMMON TORPEDO. The Torpedo, fig. 170, is a flat cartilaginous fish which resembles the common ray. Its body is smooth, and has the form of a nearly cir- cular disc, the anterior border of which is formed by two prolongations of the muscle, which are connected on each side with the pectoral fins, and which leave between these organs an oval space in which the electric apparatus is deposited. This apparatus, which is shown in fig. 171, is composed of a multitude of membranous tubes lying closely together, and sub-divided by horizontal parti- tions into small cells, like those of honeycomb, filled with mucous matter, and traversed by tbe ramifi- cations of several large trunks of the pneumogastric nerves. In tbe figure, A is the brain, b tbe spinal cord, c the eye and optic nerve, d the electric organs, e the pneumogastric nerves ramifying through this organ, f tbe branch of these constituting tbe lateral nerve, and g the spinal nerve. These organs develop electricity, which is identified in all its physical properties with that of the voltaic apparatus. The torpedo, though less powerful than the gymnotus, is capable, nevertheless, of rendering insensible the arms of those who touch it. It has been lately ascertained that the electric functions of these organs have a close connection with the posterior lobe of the brain, since by destroying this lobe, or dividing the nerves which proceed from it, the animal is deprived of the electric power. There are several species of the torpedo which inhabit the seas which FISHES. 189 wash the coast of Europe. They have been frequently found near the shores of Vendee and Provence in France. Fig. 171. 230. The Silurus Electricus, fig. 172, another of these species, which is found in the Nile and Senegal, has a length Fig. 172. of from twelve to sixteen inches. The seat of its electric power 190 ANIMAL PHYSICS. seems to be a particular tissue situate between the skin and the muscles of the sides, having the appearance of a foliated cellular tissue. The Arabs give to this fish the name Raasch, an Arabic word which signifies thunder. 231. Species of electric fishes.— Of electric fishes the seven folio win- genera have been enumerated : — 1. Torpedo narke risso. 5. Silurns elect ricus. 2. ■ unimaeulata. 6. Tetraodon eleetricus. *;• ’ niarmorata. 7. Gymnotus eleetricus. 4. galvanii. No observations sufficiently exact and extensive have yet supplied the data necessary to determine the source of the vast quantities of ele^triritv which these creatures are capable of developing at will. There is nothing in the phenomena observed which countenances the supposition that the electricity is the result either of mechanical, thermal, or chemical causes. When it is therefore stated to arise from a physiological action peculiar to the organisation of the animal, a name is merely given to an unknown agency. In the absence, therefore, of any reasonable theory, we are compelled to limit ourselves to a mere statement of the phenomena. According to the observations of Walsh, who first submitted this anim-il to exact inquiry, the following are its effects : — If the finger or the palm of the hand be applied to any part of the body of the animal out of the water, a shock will be felt similar to that pro- duced by a voltaic pile. If, instead of applying the hand directly, a good conductor, such as a rod of metal several feet in length, be interposed, the shock will still be felt. If non-conductors be interposed, the shock is not felt. If the continuity of the interposed conductor be anywhere broken, the shock is not felt. The shock may be transmitted along a chain of several persons with joined hands ; but in this case the force of the shock is rapidly diminished as the number of persons is increased. In this case the first person of the chain should touch the torpedo on the belly, and the last on the back. When the animal is in the water, the shocks are less intense than in the air. It is evident that the development of electricity is produced by a volun- tary action of the animal. It often happens that in touching it no shock is felt. But when the observer irritates the animal, shocks of increasing intensity are produced in very rapid succession. WTalsh counted as many as fifty electrical discharges produced in this way in a minute. In a series of observations and experiments made on the torpedos of Chioggia, near Yenice, by MM. Becquerel and Breschet, it was ascertained that when the back and belly were connected by the wires of a sensitive reoscope, a current was indicated as passing from the back to the belly. They also found that the animal could at will transmit the current between any two points of its body. In a series of experiments made on the torpedos of the Adriatic, M. Matteucci confirmed the results obtained by Mil. Becquerel and Breschet, FOSSIL FISHES. 191 and also succeeded in obtaining the spark from the current passing between the back and belly. 232. Fossil Fishes are more numerous than all the other vertebrated animals found in the fossil state taken together. Thus while only 400 fossil species of mammifers, 66 of birds, and 276 of reptiles have been discovered, not less than 1000 species of fossil fishes have been described. These are dis- ti'ibuted in all the sti'ata, from the first appearance of animal life in the lowest group to that which immediately preceded the actual epoch. As might be expected, however, the cai-- tilaginous fishes ai'e much more rare thau the osseous. Like those of reptiles, the skeletons of osseous fishes are often found complete, the bones having retained their proper arrangement and juxtaposition, and the body being sometimes still covered with its scales. Separate bones, and especially heads, are frequently seen, but the pai-ts which offer most resistance to destructive agencies are the teeth and small internal bones of the head, all of which are found in strata from which all other bones have disappeared. In fig. 173 is shown a group of fishes, Lebias Cephalotes Fig. 173. LEBIAS CEPHALOTES. (FOSSIL.) found at Aix, in Provence, in the tertiai-y strata. This was a fresh-water species, several genera of which still exist. 192 ANIMAL PHYSICS, In fig. 174 is shown a specimen of the family plataxidae (squamipennis). In this family the base and even the spinous part of the dorsal fins are covered with scales. The body is Fig. 174. PLAT.E ALTISSIMUS. (FOSSIL). compressed. Of ten genera which have been found in the fossil state, three are extinct. INVERTEBRATE ANIMALS. 193 INVERTEBRATE ANIMALS. 233. The presence or absence of a vertebral column ancl internal skeleton is the most conspicuous mark of distinction among animals, and all naturalists agree in adopting it as the basis of the division of the animal kingdom into two principal groups, called the vertebrate and the invertebrate. 234. The invertebrate animals, while they are generally less in magnitude, are infinitely greater in number than the vertebrate, some of then- numerous genera consisting of some thousands of species. They may be distributed, as regards their general appearance, into three principal groups ; the first including the animals of Annulose structure; the second, Mollusca — a Latin word, signifying soft — which includes those animals whose bodies are soft, and not annulose or articulated, some of which are enveloped in shells, such as snails and oysters, while others are destitute of that covering ; and the third, Zoophytes, being the lowest grade of the animal kingdom, which takes its name from two Greek words — (,(iiov (zoon), an animal, and for example, is fractured, the animal throws off the broken limb, after which the haemorrhage ceases and a new limb begins to make its appearance, the growth of which is at first slow ; a r V THE PRAWN. Fig. 206. a , first pair of antennas ; ai, second, or inferior pair ; l, lamellated appendage, covering their base ; r, rostrum, or frontal pro- longation of the carapace ; y, eyes ; pm, external feet jaws ; p', first pair of thoracic legs ; p", second pair ; fp, false abdominal legs, endowed with the functions of swimmers ; n, caudal swimmer. but becomes more rapid after the next moult, and soon assumes its full proportions. 274. Fossil Crustacea, like reptiles and fishes, are found in all the geological periods since the first appearance of animal life on the earth. About forty genera have been found in the lowest strata of the secondary rocks, two in the trias, thirty-six in the oolite, six in the cretaceous, and twenty-four in the testaceous rocks ; from which it would seem that the prevalence of the Crustacea has been decreasing since the earliest geological epochs. Such a conclusion would, however, be opposed to the fact, that the number of genera existing amounts to two hundred. Among the mast remarkable of the extinct Crustacea are the trilobitcs, an order which consisted of au oblong body, divided transversely into three parts, and also longitudinally into the same number of lobes. The com- parison of the forms of these animals with those of existing Crustacea * D'Orbigny. 220 ANIMAL PHYSICS. render it probable that they dwelt in the depths of the sea far from coasts oa mg on their back, and never resting, inasmuch as their feet could not letarn them stationary, and movement was necessary for their respiration. Fig. 207.* 0GYG1A GUKTTARDI. They lived in large troops, and their existence was limited to the Silurian period ; after which they disappeared. In fig. 207 is shown a specimen of this order — the Ogygia guettardi. WORMS. 275. This class, denominated Annelids, is characterised by a peculiar nervous and vascular system. Its types are the com- mon earthworm and the leech. The body is in general very soft, and divided transversely by numerous wrinkles or grooves. Sometimes there is a distinct head, but oftener that part is confounded with the body. The annulose segments are in most species supplied D’Orbigny. ♦ Edwards. WORMS. 221 with the analogues of legs in the form of hooked bristles, fig. 208, which serve the animal for locomotion, and sometimes for defence. In the case of annelids, such as leeches, fig. 209, which are destitute of those bristles, there is a sucker at each extremity Fig. 209.* THE LEECH. of the body, which, combined with the contractile power of the body, serves for locomotion. Several of the sea-worms provide for themselves a habitation consisting of a long tube, formed of calcareous matter secreted by their proper organs, and sometimes formed of particles of sand or gravel aggluti- nated together. Examples of the former are presented in the case of the twisted tubes which are seen attached to dead shells dredged from the bottoms of arms of the sea. These tubes are the habitations of sedentary worms, called Serpulm (fig. 210). If the tubes be placed in a vessel of sea-water while the animals still live, a curious and pleasing spectacle will be witnessed. The mouth of the tube will be first seen to open by an exquisitely-constructed door, which will be raised by the creature, from which it will cautiously protrude the head and anterior part of its body, spreading out at the same time two gorgeous, fan-like expansions, of a rich scarlet or purple colour, which float elegantly in the surrounding water, and serve as organs of respiration. 276. Leech. — The instrument of suction of the medicinal leech consists of small semi- circular horny saws, so arranged radially that their edges are presented to a common centre. No sooner is this sucker implanted in the skin, than the mouth becomes tightly everted, and the edges of the saws thus made to press upon the tense integument, a sawing movement being at the same time given to each, so that they cut their way to the sluices of blood beneath. Nearly the entire body of the animal consists of a series of chambers, into which the blood thus taken is received. These are eleven in number, perfectly distinct ; and in the first eight, the blood may Jones, Natural History, p. 313. 222 ANIMAL PHYSICS. remain for months unchanged in colour or fluidity, the creature merely allowing so much to pass into the alimentary canal as is necessary to preserve its existence. Hence the repugnance of the animal to repeat the operation until the store of food with which it has been gorged is consumed.'’ 277. Fossil Annelids of the genus of Serpube are found in all the rocks, from the Devonian upwards, fig. 211. Remarkable Fig:. 211. SERPULA FLAGELLUM. imprints of the dorsi-branchial order have been discovered in the Silurian rocks, a specimen of which is shown in fig. 212. In connection with this division may be mentioned the Helminthes or Nematoids, and the Cestoids or Taenoids, intes- tinal worms which live as parasites in the bodies of other animals, and are thence called by the generic name Eutozoa. these the tcenia, or solitary worm of the human body, is an example. Their organisation presents no character of interest to claim any extended notice in this work. ROTIFER A. 278. This division, one of the species of which is well known * Jones, Natural History, vol. i. p. 322. Owen, Lectures, p. 133. 1 D’Orbigny. KOTIFEKA 223 as the wheel animalcule , was classed among the Infusoria, and supposed to be a mere mass of animated jelly, until the improved powers of the mi- croscope displaying its true structiu'e, proved it to he a true annelid, of microscopic minuteness. Tlie body, which is translucent, is shown highly magnified in fig. 213. The mouth is placed at the anterior extremity, round which are ranged vibratile cilia, the rota- tory motions of which produce the eddies of water indicated in the figure. The mouth is surrounded by powerful muscles, and armed with lateral jaws. The alimentary canal is straight, and, extending from one end of the body to the other, has an enlargement near the middle, which constitutes the stomach. Various muscles, sali- vary glands, ovaries, and a nervous apparatus, have been discovered in the organisation of these minute creatures. The rotifera feed upon a still more minute tribe of animals called polygastrica. They possess an instrument by which they can attach themselves to one spot, and feed at ease upon the nutriment which the eddies produced by the cilia surrounding the mouth con- stantly draw into it. They are remarkable for their tenacity of life. Spallanzani kept Fig 213. them in a state of torpor for four the rotifera. years, after which, being immersed in water, they recovered their vitality. He further showed that they could be revived, after a long succession of torpid intervals, by alternately drying and moistening the same individual. He tried this experiment sixteen times on the same assemblage of animalcules. After such inhumation, a gradually decreasing number were restored to life ; and after the sixteenth none revived. MOLLUSCA. 279. This division of the Animal Kingdom, which takes its name from the softness* of the body common to all tho individuals Molluscus, soft. 224 ANIMAL PHYSICS. which compose it, is distinguished from the superior orders by the absence of either the internal skeleton of the vertebrates, or the tegumentary skeleton of animals of annulose structure. The body is sometimes naked and sometimes covered with a shell. It is distinguished from the animals below it in the scale of organisation by the symmetrical distribution of its organs round a median plane. In popidar use, the term shell-fish is applied indifferently to certain Crustacea and mollusca, — to the lobster, for example, and the oyster. The shelly envelope has, however, a totally different character in the two cases. In the Crustacea the shell is the skin rendered horny or calcareous. In the Mollusca the shell, being no part properly of the body, is the habitation or house in which the animal dwells. 280. Classification. — Mollusca are distinguished also from other animals superior and inferior, as well as from each other, by certain peculiarities in the nervous and vascular systems which will be noticed hereafter. Mollusca are distributed into two classes, according to the structure of their shells. Those of which the shell consists of a single piece are called univalve, and those which have a shell composed of two similar pieces united by a sort of hinge, are called bivalve. The snail is an example of the former, and the oyster of the latter. Mollusca, properly so called, are also resolved into two classes — one having a head distinguishable from the body, and the other in which no head is apparent. The former are called encephala, and the latter acephala. The Encephala. — The head is supplied with appendages and eyes ; and the animal, when it possesses a shell, is invariably univalve. The class is sub-divided, according to the position and arrangement of the members of locomotion, into cephalopodes, having the feet surrounding the head ; p teropodes, having the feet like wings at each side of the neck, their form being that of paddles or fins ; and, in fine, the yasteropode s, of which the members, having either the form of feet, or of a fleshy disc, are placed upon the inferior surface of the body. The Acephala, in which there is no distinct head, are always supplied with a bivalve shell. Besides these, are two subdivisions called the tunicata and the ciliated polypes ; the former having a heart and vascular system, of which the latter is destitute. 2S1. Cephalopodes exhibit the most fantastic varieties of form. The head, placed in the inferior surface of the body, and surrounded by the feet, tentacula, or organs of locomotion, is trailed aloDg while the animal moves. An example of this class is presented in the common poulpe or octopus, fig. 214, where the inferior surface is shown, the eyes presenting a remark- MOLLUSCA. 225 able appearance. There are here four pair of tentacula symmetrically dis- posed round the head, which serve equally as members of locomotion and prehension. By these organs the poulpe is rendered a most destructive Fig. 214.* THE COMMON POULPE OR OCTOPUS. animal, for neither superior strength, nor agility, nor defensive armour, can save its victim. Not fewer than 120 suckers, as efficacious as the surgeon’s cupping-glass, are disposed around each of the eight tentacula. IV lien the poulpe only touches its prey, so that a few of these suckers take effect, there is no escape for it. Its swiftness, however great, is unavailing, since it carries its enemy with it. The shell of the crab or lobster is an ineffectual defence, for it is easily broken in pieces by the hai-d and crooked weapons of the poulpe. + The cuttlefish, another example of the cephalopodes, has five pair of tentacula. In some species, as, for example, the Argonaut , or paper nautilus, fig. 215, one pair of these is terminated in a membranous enlargement, which was supposed to be used as a sail when the animal floats on the surface, while the remaining pairs served the purpose of oars. This, however, pretty and attractive as the supposition is, appears to be unfounded. The animal never uses its tentacula for such a purpose, and moves through the water backwards, like other cuttlefish. 282. Fossil Cephalopodes are exceedingly numerous ; but of all the genera hitherto discovered, one only, that of the nautilus, has come down to the present times. These fossils appear in great numbers in the lower strata of the secondary rocks, are few in the lias and oolite groups, * Edwards. f Jones' Outlines of the Animal Kingdom, p. 431. Q 226 ANIMAL PHYSICS, re-appear in great numbers again in the cretaceous, and dis- appear once more in the tertiary rocks. Fig. 215.* THE ARGONAUT, OR PAPER NAUTILUS. The examples are so numerous, and preserved in such perfection, that it is difficult to select any in preference to another, as illustrations of their forms. The Nautilus, the only surviving genus of the tentaculiferous cephalopodes in the first periods of animal life, had nearly the form * Edwards. MOLLUSCA. 227 Fig. 216.* NAUTILUS DANIANS (Fossil). Fig. 217* CERATITES NODOSU3 (Fossil). Fig, 218.* AMMONITES HUMPRIESIANUS (Fossil). * D’Orbigny. 228 ANIMAL PHYSICS. which it still retains, fig. 216. The Ceratites, fig. 217, and Ammonites, fig. 218, are familiar to all geological amateurs. 283. Gasteropodes are provided with a head, and moved by a fleshy disc or foot, or by a swimmer or fin placed under the belly. This class consists chiefly of animals lodged in an uni- valve shell, having the form of a conical spiral, of which the common snail is the type ; but some species, of which the slug is an example, are without that covering. The body is elongated, with a head in the front part, having from one to six pair of fleshy tentacula. The back is covered with a mantle which extends backward, in the form of a membraneous sac, the edge of which secretes the matter which forms the shell. The belly is covered by the fleshy mass of the foot. This class includes snails, limpets, chitons, the vermetus, the murex, ke. 284. Fossil Gasteropodes are, like tbe Cepbalopodes, extremely Fig. 219. MUROHISONIA BI GRANULOSA.* Fig. 220. CTPR.EA ELEGAXS.* numerous. The terrestrial and fluvial genera have in general appeared for the first time in the tertiary period ; but the marine genera have been found in all the rocks from the silu- D'Orbiguy. MOLLUSCA. 229 rian upwards, and in gradually increasing numbers. They were, therefore, among the earliest manifestations of animal life on the globe ; and what is remarkable is, that most of the genera, including even those of the Silurian period, still survive. The close analogy of these ancient forms with the existing species will be manifest by some examples taken from among the countless numbers of fossil shells collected by geologists. A fossil shell from the permian group is shown in fig. 219, and one found in all the tertiary beds in fig. 220. One of the genera which first appears in the middle strata of the creta' ceous group is shown in fig. 221, and one which begins in the middle of ,the oolite, in fig. 222. 285. Pteropodcs are sometimes provided with a shell, and D’Orbigny. 230 ANIMAL PHYSICS. sometimes destitute of that covering. Their structure presents but little interest. 280. Acephala, of which the oyster is the type, have a body enveloped in a mantle, as a book is enclosed by its binding. The skin connected only on one side, forms a large fold which to' a i f r- Fig. 223.* INTERNAL STRUCTURE OF THE OYSTER. envelopes all the organs of the body, and is sometimes united so as to leave openings only before and behind, to allow the entrance and escape of the water necessary for respiration. The body is enclosed in a bivalve shell, hinged by an elastic ligament, and opened and closed by the action of suitable muscles. The structure of the oyster is shown in fig. 223, where v is one of the valves of the shell ; v', the hinge ; m, one of the lobes of the mantle ; m', part of the other lobe, folded back ; c, adductor muscle ; br, bran- Fig. 224.* cilia or gills; b, mouth; t, tentacula; /, shell OF THE pearl ovsTER. ];ver . ^ intestines ; o, anus ; b, heart. Fig. 224 shows the shell of the pearl oyster. The Acephala are resolved by naturalists into two orders : 1°, the LameUibranchcs, which include the oyster, the mussel, the scallop, and the cockle ; 2°, the Brachiapodcs , or arm-footed, which derive their name from two fleshy arms which replace the feet, of which the terebratula is an * Edwards. MOLLUSC A. 231 example. This latter order consists of few existing species, and these dwell at great depths in the sea, being generally brought up from depths of sixty to one hundred fathoms. Professor Owen remarks, in reference to the Brachiapodes, that their respiration and nutrition at such depths are subjects suggestive of interesting reflections, and lead one to contemplate with less surprise the great strength and complexity of some of the minutest parts of the frames of these diminutive creatures. In the unbroken stillness which must pervade those abysses, their existence must depend upon their power of exciting a perpetual current around them, in order to dissipate the water already laden with their effete particles, and to bring within the reach of their prehensile organs the animalcules adapted for their sustenance. 287. Molluscoids form a subdivision of Mollusca, consisting of two groups, the Tunicata and the Bryozoares. The former, instead of a shell, are invested with a sort of leathery covering, or tunic, from which they take their name. The latter have a less per- fect covering or mantle, the gills or branchiae being uncovered. 288. The Tunicata have a vascular system which is attended with the peculiarity of an alternate change of direction of the circulation, so that the vessels which are arteries at one time are veins at another, and vice versa, a circumstance which will be better understood when the reader has studied our chapter on the vascular system. In this class are included the biphora, the pyrasomes, and the ascidia. 289. The Bryozoares -which, until recently, had been con- c b u. Fig. 225. PLUM ATKI. I./K. * founded with the most simple Polypes, have organs of aquatic respiration, * Edwards. 232 ANIMAL PHYSICS. consisting of a circle of tentac-ula around the mouth, which are supplied laterally with vibratile cilia, as may be seen in the example of the plumatella, fig. 225 ; a , a group of plumatellse in their natural size ; o, others magnified, and seen in different positions ; c, the anus. In this case there is circulation without a heart, the blood moving between the viscera and the mantle. The inferior part of the mantle is in general hardened, so as to form a sort of tube or cell, sometimes of horny and sometimes of calcareous consistency, into which the animal can retire. In general these animals, which are of almost microscopic minuteness, live assembled in groups more or less considerable. They mostly inhabit the sea, but some of them are found in fresh water, among which are included the plumatella, common enough in ponds. 290. Fossil Acephala are infinitely great in number and va- rious in form, and prevail through all the geological periods, from the silurian to the uppermost tertiary. Of the Lamellibranchhe most of the genera still survive, a few only becoming extinct in each group of rocks. A less proportion of the Brachiopodes and Bryozoares survive. In all, however, the number of species extinct is very great. How closely these molluscs of the earlier epochs of the globe resembled in their structures the living species, will be seen by reference to a few of the numerous examples of fossil species which have been reco- vered, 291. The Spondylus, fig. 226, of which there are forty-five fossil Fig. 226.* SPONDYLUS SPINOSCS. species, first appears in the lowest stratum of the cretaceous group, and presents an example of the fossil Lamellibranehise. 292. The Fentamerus. fig. 227, of which there are twenty-one fossil species, is an example of the Brachiopodes. This species is first seen in the lowest strata of the silurian group, and becomes extinct after the * D'Orbiguy. MOLLUSCA. 233 Fig. 228.* RETICULIPORA OBLIQOA. * D’Orbigny. 234 ANIMAL PHYSICS. devonian period, so that its existence was limited to the earliest epochs of annualisation. 293. The Reticulipora, fig. 228, an example of the Bryozoares, is aa extinct genus Retepora, of which there are five fossil species known ; the first in the middle strata of the oolites, and the others in the upper strata of the cretaceous group. In this the meshes are formed of high vertical lamina;, supplied with cells by transverse lines on each side ; «, shows the whole in its natural size ; b, the external part magnified ; c, the internal part magnified ; cl, the laminae as shown with a still higher magnifying power. ZOOPHYTES. Fig. 229. HOLOTHURIA. times supported by a sort of solid skeleton, fig. 229, the internal 294. This being the lowest branch of the animal kingdom, may be regarded as constituting the transition from animal to vegetable life, from which character it has received its name. The organs, instead of being disposed symmetrically on each side of a median plane, are grouped around an axis, or centre, so as to give to the body a radiated or spheroidal form. The peculiarity of the nervous system will be explained hereafter. A great variety of structure prevails among these animals, many species of which more resemble vegetables than animals. According to these varieties of form, they are distributed in five classes : 1°, the Echinodermata ; 2°, the Acalepha ; 3°, the Polypes ; 4°, the Polygastria ; and 5°, the Sponges. The Echinodermata are animals having a thick skin, some- . D’Orbiguy. ■ ZOOPHYTES. 235 structure of which is very complicated. They are formed to Fig. 230. THE SEA-URCHIN. creep at the bottom of water ; most of them, like the sea- Fig. 231. THE STAR-FISH. urchin, fig. 230, have a digestive tube, open at both ends ; 236 ANIMAL PHYSICS. but with some, as the star-fish, fig. 231, the food is received, and the excrement discharged, through the same opening. These animals consist of three principal groups — the holothuria, fig. 229, the sea-urchin, fig. 230, and the asterias, or star-fish. fig. 231. In the figure of the sea-urchin, the spines have been re- moved from the left side to display the structure of the shell 295. The Acalepha, or Sea-nettles, are animals having a body of gelatinous consistency, organised for swimming in the sea. The organisation is reduced to the most simple form, consisting of little more than a stomach, having only one opening for reception and discharge, and having canals ramifying to diflferent parts of the body. 296. Medusa.— The family of this class which is best known is the Medusa, fig. 232, in- cluding the rhizostomes, which abound on all the coasts of Europe. What is most remarkable in the structure of this animal is the stomach, which communicates with the exterior, not by a mouth or anus, but by a great number of small tubes, terminated by pores at their extremities. The Medusae are ovi- parous, but the young, which issue from the egg, bear no resemblance to the parent. They are small oval bodies, the surface of which is covered with vibratile cilia. These soon take the form of the hydroida, which reproduces by means of germination, or buds, and the multitude of animals thus produced detaching themselves successively, assume ultimately the form of the Medusa, 297. Coral Animals, or Polypes, have bodies cylindrical, ZOOPHYTES. 237 soft, 'with a mouth at the extremity surrounded by tentacula, which serves equally for the reception of food and the ejection of excrement. It leads into a great cavity which occupies the entire body, and which contains the ovaries attached to its sides, and is continued upwards to the tentacula. The inferior part is so constituted as to adhere to bodies, fixed upon which the animal is destined to live. The skin is for the most part hardened, so as to form a horny or calcareous envelope analo- gous to the cells of the Bryozoares already described. These Polypes resemble the Molluscoids also in their mode of propagation, being multiplied as well by germination as by eggs, so that successive generations remain fixed one upon another, forming masses more or less considerable, which include multitudes of individuals of the same race, and having as it were, a common vitality. The part of the tegumentary covering of these animals which is hardened forms sometimes tubes, and sometimes a Fig. 233. POLYPES OP THE OENU3 ASTEROIDES. species of cells, with which every one is rendered familiar by corals. These were long regarded as merely the habitation of the animal. 298. Coral Reefs and Islands — Sometimes each polyp posesses a distinct tube, or cell ; but, generally, it is common to a mass of the animals. It is thus that polypes, the bodies of which individually, do not exceed some inches in length, raise 238 ANIMAL PHYSICS. coral reefs and islands in the tropical seas. Under circum- stances favourable to their development, these animals com- pletely cover chains of rocks and extensive banks, so as to produce masses of constantly increasing magnitude by the in- cessant accession of new bodies as the animals multiply. The solid remains of such a colony continue after these frail races have perished successively, and serve as the foundation upon which succeeding tribes erect new structures, until at length the accumulation rises above the surface of the ocean, and forms an island. The island thus formed, soon becomes the theatre of phenomena of another order. Decaying animal and vegetable matter, resting on it, form a soil. Vegetable seeds carried by the winds and the waves germinate there, and speedily cover the surface with a rich vegetation, until at length these vast cata- combs of almost microscopic zoophytes become habitable islands. In the Pacific, numerous islands thus produced are found. In general they seem to have been raised upon the craters of extinct volcanoes, since they have almost invariably a circular Fig. 234. CORAL ISLAND IN THE PACIFIC. form, and present in the centre a lake communicating by a channel with the surrounding ocean. Some of these island-lakes are thirty miles in diameter (fig. 234). 299. Infusoria are microscopic animalcules developed in countless numbers by decaying organic matter, immersed in water. Until recently, the wheel-animal has been erroneously ZOOPHYTES, 239 classed with these. Their bodies are round, or elongated, Fig. 235. PENTACRINU8 FASCICULOSU8. covered with minute cilia, and show in the interior many small 240 ANIMAL PHYSICS, Fig. 236. CYATHINA BOWERBANKtI. Side view. Fig. 237. ANAEACIA ORBULITES. Upper surface. D’Orbigny. Lower surface. ZOOPHYTES. 241 cavities which have the properties of stomachs. Sometimes these communicate with a canal having two open extremities, but often no communication with the exterior is apparent. Natural- ists are not agreed as to their mode of propagation. It was formerly thought, and is still believed by some, that they can be produced directly, by the decomposition of vegetable and animal matter : this, however, is very doubtful, and it is certain that, at least in the case of some species, they are pro- pagated one by another. It is certain, however, that their reproduction is in accordance with the simplicity of their structure ; since in most cases, these singular creatures are mul- tiplied by the spontaneous division of their bodies, into two or more parts, each part becoming an individual similar to that from which it is separated. 300. Fossil Zoophytes of every order are found in count- less numbers, in all the strata from the silurian upwards. Their forms are analogous to those of the existing species, although multitudes of the genera, and still greater multitudes of the species, have disappeared, many having become extinct in the earliest geological epochs. Of the vast numbers of specimens of every order supplied by the researches of paleontologists, we must here limit ourselves to a few. 301. The Pentacrinus Fasciculosus. — Fig. 235 is a spe- cimen of the genus of the radiated Zoophytes, of which thirty- seven fossil species are known. They first appear in the upper strata of the trias, and are most numerous in the Oxford clay of the oolite group. The living species inhabit great depths in the seas of the West Indies. 302. The Cyathina.— Fig. 236 is a specimen of the fixed Polypes, of which five fossil species are known. They first appear in the upper strata of the cretaceous group, and increase in number upwards. 303. The Anabacia. — Fig. 237 is a genus of the free polyp, of which three fossil species are found in the middle and lower strata of the oolite group. n 242 ANIMAL PHYSICS. CHAPTER V. THE NERVOUS SYSTEM. 304. The nervous system is the link between mind and matter. In all animals it connects instinct and intelligence with the organism, in man especially the soul with the body. There is no part of the corporeal structure whose empire is so general and extensive. So essential is its supremacy that its suppression involves that of life itself. It is the recipient of impressions from without, and the messenger of dictates from ■within, and constitutes therefore the means by which the individual is placed in communication with the external world. Its develop- ment is the unerring measure of elevation in the scale of intelligence, and it is accordingly the organ by which the supe- riority of the human race over all the kingdoms of nature is manifested in the most strildnyf manner. The control of the nervous system, not limited to those powers which have relation to sense and will, extends equally to the functions whose exercise is independent of conscious- ness, which are even more essential than sensation itself to the conservation of ■vitality. Thus its dominion over circulation and nutrition is as absolute as over feeling and volition. While it executes the commands of the will in general, it reacts on the will in all special cases where the exigencies of the organism are imperious, and reacts moreover with an energy always proportionate to the necessities of the case. It is thus that the pain which it causes to be felt is at once the notice of local injury or disease, and the stimulus to supply and redress orgauic want and derangement. 305. In man and all animals of analogous structure, the nervous apparatus consists of two parts which, though not altogether independent of each other, are so distinct in their functions that it is convenient for the purposes of exposition to explain them under different heads. The first is denominated the cerebrospinal and the second the ganglionic system or great sympathetic nerve. The former presides over the functions of sensation and volition, the latter over those of nutrition and circulation. CEREBROSPINAL SYSTEM. 243 306. Each system consists of central and circumferential parts ; central, from which the nervous influence diverges to the circumferential, or circumferential, from which it con- verges to the central. The central parts consist of masses, more or less considerable, of nervous matter collected in certain regions of the organism, the substance and form of which varies from point to point. The circumferential or radiating parts consist of innumerable cords, called nerves, issuing in trunks from the central parts and diverging into innumerable ramifications, which becoming more minute as their number increases, are spread over all the organs of the body. 307. These two systems have been sometimes denominated, from their respective functions, the nervous system of animal life and the nervous system of organic life. THE CEREBRO-SPISTAL SYSTEM. 308. The central part of this system is a soft mass of whitish and greyish matter, differently disposed in different parts, in the cavities of the skull and the vertebral canal, and throwing out trunks through the numerous foramina in the bony walls of these. This mass is called from its position in the skull and spine, the cerebrospinal axis. 309. That part which occupies the skull is called the encephalon, from the Greek word cyKtfaXos (enkephalos) in the head. That part which is deposited in the vertebral canal is called the spinal cord or sjrinal marrow. The encephalon consists of several distinct parts, differing in form, substance, and functions, the principal of which are the cerebrum, the cerebellum, and the medulla oblongata. 310. The nervous matter composing the cerebro-spinal axis, besides the protection it receives from the bony casing in which it is deposited, is enveloped within this casing by three mem- branous coats, placed one within the other, called the dura mater, the arachnoid, and the pia mater. 311. Dura Mater — The dura mater, which is the exter- nal coat, is a strong, thick, fibrous membrane. It adheres in many places to the inner surface of the cranial bones and descends in folds into the fissures which separate the large divisions of the cerebral matter, thus forming between them R 2 244 ANIMAL PHYSICS. a partition which prevents their displacement, and protects them from any undue mutual pressure which might attend the ever varying position of the body and its members. 312. Arachnoid. — The arachnoid (apa^irq, cobweb) membrane, so called from its resemblance to a spider’s web in its texture, is the second coating. Part of it is in immediate contact with and, inseparable from the dura mater, which has so far the character of a fibro-serous membrane. A space intervenes between the arachnoid and the pia mater, filled with a liquid called the cerebrospinal fluid. This fluid, submitted to analysis by M. Lassaigne, was found to consist of 984- per cent, of water combined with 0'8 per cent, of common salt and chloride of potassium, with very small pro- portions of osmazome, albumen, phosphate of lime, and car- bonate of soda. Physiologists are not agreed as to its origin. According to Cruveilhier it is secreted by the arachnoid, and according to Haller, Magendie, and Longet, by the pia mater. Its total quantity weighs about 2T2 ounces, but varies according to age and sanitary condition. Its use is to fill the vacant spaces which exist between the differ- ent parts of the cerebral matter, such as the anfractuosities and fissiu-es which divide the convolutions of the brain, and also the space which intervenes between the dura mater and the nervous centres. It may therefore be considered as discharging a function similar to that of a lubricant, preventing the inju- rious contact, friction, and pressm-e which might attend the changes of position consequent upon the flexibility of the ver- tebral column, and the mobility of the head. 313. Pia Mater. — The pia mater, in immediate contact with the nervous matter, is a cellular welt, having but little con- sistency, in which an infinity of minute and tortuous blood- vessels are ramified and interlaced in a thousand different directions. One of the uses of this envelope is supposed to be to moderate the force with which the blood is propelled through the delicate cerebral structure. It would seem to act after the maimer of a breakwater. 314. A theoretical illustration of the general form and dis- position of the cerebro-spiual system is given in fig. 23S. — a Fig. a. — Cerebrum. b. — Cerebellum. c. — Spinal cord. d. — Facial nerve. e. — Brachial nerves. /. — Median nerve. 9. — Ulnar nerve. A. — Internal cutaneous nerve. 23S. t. — Intercostal nerves. k. — Lumbar nerves. l. — Sciatic plexus. »». — External peroneal. «. — Tibial nerves. "• — External peronoal nerve r— External saphenc. 246 ANIMAL PHYSICS. 315. Encephalon. Tlie relative magnitude, position, and connection of the principal parts of the encephalon are shown by a side view in outline in fig. 239 ; the parts being a little removed from their natural juxtaposition to render their con- nection more apparent. It will be seen that the cerebrum, or brain properly so called (a), constitutes by far the largest portion of the encephalic mass, extending over the entire skull, from the eyebrows to the back of the skull, and from ear to ear. It covers the cerebellum (b), which, in the figure, is a little re- moved from it, but in the natural position close to it ; a fold of the dura mater, however, intervening. The spinal cord, terminating at the foramen magnum, is continued, under the name of the medulla oblongata (d), into the skull. The connec- tion between the cerebrum and the medulla oblongata is made by two peduncles , or stalks, one of which (a) appears in the figure concealing the other behind it. These, diverging from each other upwards, are lodged and lose themselves in the cerebrum. Three pair of peduncles issue from the cerebellum ; one (b) directed upwards to the cerebrum, another (d) downwards to the medulla oblongata, and the third (c) forwards, being connected by a band of cerebral matter (c) continuous with ENCEPHALON. 247 them passing in front of the peduncles, and called the pons Varolii (c),* being as it were a bridge passing over the junction of the peduncles and the medulla oblongata. It appeal's, therefore, that the principal parts of the en- cephalon are the cerebrum (a), the cerebellum (b), the pons Yarolii (c), the cerebral peduncles (a), the peduncles of the cerebellum ( b , c, d), and, in fine, the medulla oblongata (d) ; the peduncles, however, being considered as connections rather than chief parts. 316. If the total mass of the encephalon, exclusive of the peduncles, be expressed by 10000, the mean proportion in which it will be distributed between the parts is as follows : — Cerebrum . . . . . 8756 Cerebellum ..... 1045 Pons varolii, and medulla oblongata . . 199 10000 Thus, it appears than in man the cerebrum, or brain proper, constitutes 87§ per cent, of the whole encephalon. This proportion of the cerebrum, which, as null be shown hereafter, is the exclusive seat of intelligence, to the other parts of the encephalon, is much greater in man than in any of the inferior species. 317. The absolute mean weight of the brain, separated from the subordinate parts, according to the observations of Cruveil- hier, is 44 '1 ounces, the limits of its variation being, in the adult, 53 ounces and 35 ounces. M. Parchappe found nearly the same average weight of the cerebrum in the case of twenty- nine adult men. The absolute weight of the brain, irrespective of the total weight of the body, is greater in man than in the immense majority of animals. Three species only — the dolphin, the elephant, and the whale — present exceptions to this. The average weight of the brain of the dolphin is found to be about 63^ ounces, and that of the elephant and whale about 53 ounces. But, since the cerebellum and other subordinate parts constitute the sixth part of the whole weight, it follows that the brain properly so called of the dolphin weighs only 53 ounces, and that of the elephant and whale 44. The difference, therefore, between the gross weight of those cerebral organs and that of man is inconsiderable, while the comparison * So called from Constanzio Varoli, a celebrated Bolognese anatomist of the sixteenth century. 248 ANIMAL PHYSICS. of the weight and volume of the brain with that of the whole body gives to man an immense superiority. For while the weight of the human brain amounts to the 30th part of that of the whole body, that of the dolphin amounts to only the 100th part ; that of the elephant to the 500th part ; and that of the whale to a much smaller fraction. In the ox and the horse the mean weight of the brain Ls only 21-1 ounces, being less than half the mean weight of the human brain, while the total weight of the bodies of these animals is six or eight tunes that of the human body. 318. Numerous observations have been made with a view to determine the proportion which the weight of the brain in different classes of animals bears to the total weight of the body. Although in such observations there are accidental differences and errors in individual cases, these are effaced in taking the mean results of sufficiently numerous experiments. In pm-suing this course, M. Leuret has obtained the following results for the several classes of vertebrated animals, ascending from fishes to man. Supposing the total weight of the body to be expressed by 1 0000, the following are the average weights of the brain : — Fishes . . 1-8 Reptiles 7-6 Birds . . 47-2 Mammifers . . . 53 -S Man , # . . 277 ‘S These results demonstrate that the development of the cerebral organ is more and more considerable in ascending in the scale of intelligence. 319. This proportion which the cerebral development bears to the total weight in comparing class with class, is not so evident when we come to compare together varieties or indivi- duals. Although it has been maintained by some physiologists that the cerebral development in man varies in proportion to the stature or weight, other and higher authorities hold with Bichat that it is completely independent of the stature. Numerous observations have been made with a view to deter- mine the comparative cerebral development in the different races, according to which naturalists have classed the human species. The general result of such researches has been favour- able to the conclusion that the cerebral development is greatest in the Caucasian race, which includes the Europeans, and least in the negroes. Soemmerring found, by measurement of ENCEPHALON. 249 the skulls of the different races, that all the dimensions were less in the negro than in the European. Tiedemann, adopting a different method of determining the capacity of the skull, first weighed the empty skulls, and afterwards weighed them with their cavities filled with millet seeds, and took the difference of weights as a measime of their different capacities. By this method, applied to forty-one negroes and seventy-one Caucasian skulls, it appeared that there was no perceptible difference of capacity. But it must be observed that these methods determine, not the relative capacities of the cerebrum, properly so called, which is the exclusive seat of intelligence, but the total volume of the cerebral mass. Professor Berard accordingly measured, with great accuracy, the various dimensions of numerous well- authenticated negro skulls, which had been placed at his dis- posal, and found, in accordance with the results of Tiedemann’s observations, that the total volume of the cerebral cavities was not less than their average capacity in European skulls, but that the distribution of the space was different, a larger propor- tion being assigned to the posterior and inferior parts of the skull in the negro, than in the European. It would therefore follow, from these results, that the cere- brum, properly so called, is proportionally more developed than the subordinate parts which occupy the hinder and lower cavities of the skull, in the European than in the African race. 320. By a comparison of the cerebral organs of woman with man, M. Parchappe found that the weight of the male brain is, on an average, nine per cent, greater than that of the female. It might be maintained that this inferiority of development in the female brain is merely a consequence of the inferior dimen- sions and weight of the female body ; and some physiologists, including Meckel, have even maintained that the female brain, relatively to the total dimensions of the body, is greater than the male. Against this, M. Parchappe maintains that the whole encephalic mass, which is less in woman by one-eleventh than in man, is not sensibly greater, relatively to the whole mass of the body, and therefore that its absolute inferiority is not compensated by its relative superiority. 321. Of all questions connected with cerebral development, the most interesting by far are those which involve its relation to the comparative intellectual powers of individuals, but there are none concerning which the number of well established data are more insufficient. It cannot be doubted that many prominent 250 ANIMAL PHYSICS. cases are recorded of extraordinary cerebral development in remarkable men, though such cases are not always satisfac- torily authenticated. Five of the most remarkable are the following : — Cromwell 10(1 Byron . 79 Cuvier 644 Abercrombie, M.D. . . . 03 Dupuytren 50§ Of the above cases, the first and second are probably exagge- rated— the first certainly so ; the others have been well ascer- tained ; but such instances are not sufficiently numerous to supply a safe basis for any general conclusion. The most direct and certain means of deciding, by immediate observation and expe- riment, the relation between cerebral development and intellec- tual phenomena, would be the comparison of the mean weight of the brains of idiots with those of persons of sound mind M. Ldlut has, accordingly, made numerous observations with this view, the general result of which is, that the mean weight of the encephalon is less in idiots than in individuals of sane mind, and that this disproportion is much more marked in the cerebrum than in the subordinate parts. 322. Cerebrum, or Brain. — The cerebrum is divided, at its superior surface, by a fissure extending along the middle of the head, in the median plane, and occupying a position coinciding with the crest of a helmet. This channel, which commences at the lower part of the forehead, and terminates a little above the nape of the neck, is called the longitudinal fissure. The two parts into which it divides the cerebrum, which are perfectly identical and symmetrical, are denominated, without much propriety of language, hemispheres. They are, of course, convex without, and concave within, being moulded according to the form of the sides of the skull. 323. The cerebrum consists of a multitude of large vermi- form convolutions, and has the appearance of coils of thick vermicelli gathered up confusedly together. The longitudinal fissure passes quite through to the base of the cerebrum, from before to behind, completely separating the two hemispheres ; but near the middle, at a certain depth, it is traversed at right angles by a mass of white substance, called the corpus callosum. This general description will be more clearly comprehended by reference ENCEPHALON. 251 to figs. 239 and 240. Fig. 240 represents a vertical longitudinal section of the encephalon, made in the direction of the longitudinal fissure, showing in their natural position the parts, which in fig. 239 are represented dis- placed, together with other parts omitted in the latter figure. Immediately under the middle of the cerebrum is seen, in section, the corpus callosum (240, x), the upper surface of which is convex, and the under concave, and nearly parallel to it. The cerebellum lying beneath the posterior- part of the cerebrum, is shown in section, at 240, 4 ; the medulla oblongata, at 240, *, the pons Varolii, at 240, 2; and the cerebral peduncles, at 240, 3. Fig. 240.* VERTICAL SECTION OF THE ENCErHALON THROUGH THE GREAT LONGITUDINAL FISSURE. The cerebrum, divested of its membranous coverings, and viewed from above, presents the appearance of a continuous mass of vermiform convolu- tions, divided along the middle by the longitudinal fissure, and extending before and behind to the forehead and the neck, and at the sides to the ears. If the middle part of the convolutions be removed, the surface of the corpus callosum will be revealed, and will present the appearance shown in fig. 241, '. It is traversed, as will be seen, by a longitudinal groove (241, s), and parallel to this, at either side, by elevated ridges (241, 4). In its posterior part, it is formed into a slightly obtuse angle, within which are seen the edges of the superior surface of the two hemispheres of the cerebellum (241, 13), united by the vermiform process. The cerebral convo- lutions appear around the external border of the corpus callosum. 324. Tlie inferior or concave surface of the corpus callosum forms the roof of a cavity of curious and complicated structure. * From the original of Hirschfeld and Leveill6. 252 ANIMAL PHYSICS. Tlie llooi of this cavity consists of certain structures and pro- jections, to which anatomists have given more or less fanciful FRONT. BACK. Fig. 241.* THE CEREBRUM VIEWED FROM ABOVE, THE MIDDLE CONVOLUTIONS BEING REMOVED SO AS TO SHOW THE SUPERIOR SURFACE OF THE CORPUS CALLOSUM. names. These parts, taking them in the order of their position from the front towards the back of the head, are : — 1. Corpus striatum. 2. Tania semicircularis. 3. Thalamus opticus (240, 12,13). 4. Choroid plexus. 5. Corpus fimbriatum. 6. Fornix (240, S4). Some of these are shown in section, in fig. 240. 325. Septum Lucidum. — Of the two compartments into which the cerebral cavity is divided by this complex floor, the upper is again divided into two by a vertical longitudinal par- tition, coinciding in position with the median plane, called the septum lucidum, two Latin words, signifying “ a transparent * Foville. ENCEPHALON. 253 partition.” From its position, the septum luciclum coincides with the plane of the section shown in fig. 240, and is therefore represented by the space 240, 3a, included between the lower surface of the corpus callosum (240, 26) and the general flooring which dirides the entire cerebral cavity horizontally. 320. Ventricles.— The two chambers into which the septum lucidum divides the upper compartment are called the lateral cent rides, and distinguished as the right and left ; the com- partment below the flooring, not similarly divided, being called the third ventricle. To convey some idea of the complicated flooring which separates the third from the first and second ventricles, we have given in fig. 242 a hori- FRONT. Fig. 242.* HORIZONTAL SECTION OF THE ENCEPHALON, SHOWING THE FLOOR OF THE LATERAL VENTRICLES. * From Hirschfeld and Leveilld. ■254 ANIMAL PHYSICS. “““ s.ectlon ^ie encephalon, made at the level of this flooring, where -4L, -, is the corpus striatum ; 242, 2 the extremity of the section of the septum lucidum, which, as will be seen, is composed of a double lamina sepai ated to the right and to the left, so as to enclose a spindle-shaped space, in the middle of which is a small cavity, (242, '), called the fourth ventricle; 242,°, is the fornix; 242, 4, is the section of the flange, or border of the corpus callosum, which there descends to the floor of the ventricles ; FRONT. BACK. Fig. 243.* THE VELUM 1NTERP0S1TUM, LYING BETWEEN THE IORNIX AND THE TARTS BENEATH, SHOWN BY RAISING UP THE FORNIX. 242, 3, is a part called the small hippocamp; 242, 6, is the posterior pillar of the fornix; 242,', is a part called the great hippocamp, or the horn of Ammon ; 242. 8, is a part of the floor of the ventricle ; 242,'°, is the choroid plexus; 242, u, is the taenia seinicircularis ; and 242, 9, is the corpus fimbriatum. From Sappey. ENCEPHALOX. 255 327. Velum interpositum — A remarkable structure, called the velum interpositum, or choroid web, is interposed between the fornix and inferior parts. It is shown, with the adjacent parts, in fig. 243. This web (243, *), which is highly vascular, connects the two choroid plexuses, (243, -), and as well as the latter, is merely a production, or continuation, of the pia mater, which, passing from tiie outer surfaces of the brain, enters through the transverse fissure of the lateral ventricles, beneath the corpus callosum and fornix, and above the optic thalami, the quadri- geminal bodies (to be described presently), and the pineal gland. The velum has a triangular shape, its sides being the choroid plexuses (243, 2). It is traversed by several veins, such as the veins of Galen (243, 3, and 243, 5), veins of the corpora fimbriata (243, 6), of the choroid plexus (243, q) of the corpora striata and optic thalami, (243, s), of the lateral ventricle and great hippocamp (243, 9). The section of the interior pillars of the fornix is shown at 243, **, and the fornix itself turned upwards and cut trans- versely at the connection of its anterior third, with its posterior two-thirds, is shown at 243, n. The triangular depression, 243, 12, marked by oblique striae, is called the lyre. Small arteries running through the velum appear at c, b, c. 328. Base of the brain. — If the entire mass of the ence- phalon be imagined to be removed by a section made round the external borders of the skull, and to be inverted so as to present its inferior surface upwards, it will present the appearance shown in fig. 244. In this case, the posterior parts of the two cerebral hemispheres are covered by those of the cerebellum, and a complicated structure, to be presently described, appears in the centre along the direction of the longitudinal axis. 329. Lobes. — Anatomists have divided each of the cerebral hemispheres into parts called lobes. The anterior lobe (244, ') is limited by a curved fissure (244, 6), concave backwards, called the Sylvian fissure. The posterior lobes (244, 3) ex- tend from this fissure backwards to the limit of the cere- brum. This posterior lobe is divided by some anatomists into two ; but as the division is not definite and the functions unimportant, we shall here adopt the division into two lobes, anterior and posterior. The depression by which the convolu- tions are separated, called anfractuosities by foreign and sulci or grooves by English anatomists, have generally received special names, according to their positions relatively to other principal parts of the organ ; but it will not be necessary here to reproduce the nomenclature. 330. The substance composing the cerebrum consists of white matter internally, coated externally by a thin layer of 256 ANIMAL PHYSICS. greyish matter, which lias been called the cortical matter, from FRONT. Fig. 244.* INFERIOR SURFACE OF THE ENCEPHALON, DIVESTED OF ITS MEMBRANOUS COATING. its relation to the white matter being analogous to that of the bark of a tree to the wood within it. 331. Cerebellum. — The structure and substance of the cere- bellum (244 ir) differ from those of the cerebrum. The cere- bellum is not formed into convolutions, but laminated and foliated like the leaves of a book. On making a section of the cerebellum, in the direction of the median plane, a remark- able internal structure (fig. 240 4 ) is presented, called* from its peculiar appearance the arbor vita. This is produced by the extension of processes of white matter from the centre into the laminae, which consist of grey matter. 332. Besides the roots of several pairs of nerves which are * From Hirschfeld find Leveill($. ENCEPHALON. 257 cut off, tlie chief parts which are presented in the middle of the base of the brain are the pituitary body (fig. 244, 8), the corpora albicantia (244 9), the interpeduncular space (244, 10), the peduncles (244 "), the pons Varolii (244, 12), the medulla oblongata cut off (244, I3), the pyramids (244 1 '), and the olivary and restiform bodies (244, li), and (244, 16). 333. Corpora quadrigemina. — Concealed by the central parts, in fig. 244, there are some of small magnitude, but of considerable fimctional importance, which we have, therefore, shown in fig. 245. FRONT. BACK. 245. » This is a section of the cerebrum, made through the cavity of the middle ventricle, showing the surfaces of the corpora striata and the optic thalami. The parts marked a e, and the two corresponding parts on the left are called the corpora quadrigemina ; c is the corpus callosum, / the anterior Quain. 8 258 ANIMAL PHYSICS. pillars of the fornix, Z; h the corpora striata, 1 1 the optic timlami, st, * the middle ventricle, and p the pineal gland with its peduncles. The section in fig. 245 is made deeper than that shown in fig. 242. 334. The Nerves in general. — From the encephalon and the spinal cord all the nerves of the cerebro-spinal axis diverge. A general theoretical view of these is shown in fig. 240. These nerves, on either side of the axis, are perfectly similar and symmetrical ; for every nerve which issues from it to the right, there is a corresponding one, absolutely identical with it, and proceeding to a cor- responding part of the body, on the left. Indeed, this remarkable symmetry might easily be foreseen from the necessity which arises for it, from the structure of the body considered in reference to the func- tions of the nerves. If the body be imagined to be divided into two parts, right and left, by the median plane, already described, continued downwards, they will be absolutely similar and sym- metrical. With a few exceptions, to be mentioned hereafter, each member, each organ, and each vessel, on one side, has a corresponding member, organ, or vessel similarly placed on the other side of it. But since it is the function of the nerves to be the conductors of volition and sensation between the brain and all parts of the body, it necessarily follows that the same symmetry which prevails among the other parts of the organisation must likewise exist in the nervous system. Hence it follows that nerves exist in pair's, and in pairs they issue from the cerebro-spinal axis, as shown in fig. 246. 335. Anatomists have agreed to name them according to the numerical order of their roots upon the axis, commencing from the summit of the head downwards. Thus, the first pair are those which issue from the summit of Fig. 240.* * Edwards. CRANIAL NERVES. 259 the axis, near the middle of the length of the longitudinal fissure. These are also called the olfactory nerves, because they go to the organ of smelling. The second pair, the next in de- scending order, are the optic nerves, and so on. 336. Of forty-three pairs of nerves, which thus issue from the cerebro-spinal axis, twelve have their roots in the en- cephalon, and issue to their respective organs through holes properly placed in the bony case of the brain. The other thirty-one issue from the intervertebral foramina, already described, in the spinal column. The twelve which issue from the encephalon are called cranial, and the thirty-one which issue from the vertebral column are distinguished as spinal nerves. 337. Cranial Nerves. — The points from which the twelve pairs of cranial nerves issue, are all apparent in the base of the encephalon (fig. 244). The first pair are the olfactory nerves (244, w), which proceed to the organ of smelling. These issue from points at the inner extremities of the Sylvian fissures (244, *'), and in proceeding from their origin for- wards, are deposited in a depression or anfractuosity between two convolu- tions (244, la) The left olfactory nerve is divided near its root (244,2-’) to show its prismatic form, the angle of the prism being accommodated to the anfractuous groove in which the trunk of the nerve is deposited. 244, a, is the bulb of the right olfactory nerve. The second pair are the optic nerves, which decussate at a point (244, 24), immediately between the roots of the olfactory nerves. The optic nerves are cut off in the figure, near their decussation. The third pair (24 4, w) are the motor nerves of the eye, being those which govern the muscles which move the eyeball. The fourth pair are the pathetic nerves. These nerves also govern one of the muscles of the eye. The fifth pair (244, 27) are called the trifacial nerves : they are nerves both of taste and touch, imparting sensibility, by thin divergent fibres, to the face, the fore part of the head, the eye, nose, ear, and mouth. They supply motor filaments, also, to the muscles which govern mastication, and consequently have very numerous ramifications. The sixth pair (244, are called the abducent or external motor nerve of the eye. They act upon the external straight muscle of the orbit. The seventh pair (244, a) are called the facial nerves, and are the motor nerves of the face, over which they spread numerous ramifica- tions. The eighth pair (244, •i0) are the auditory nerves, the ramifications of which are distributed over the internal structure of the ear. These nerves are united with the facial nerves, as shown in the figure, by a small fibre. According to the classification of Willis, generally adopted by English anatomists, the facial and auditory nerves are taken as a single pair.° In s 2 a 260 ANIMAL PHYSICS. the classification of Soemmerring, however, they are regarded as separate nerves. The ninth pair (244, 3I) are called the glosso-pharynyeal. The terminal fibres of this pair, as their name implies, are spread over the toDgue and pharynx. The tenth pair (244, 32) are called the pneumogastric nerves. The rami- fications of this pair, which are the longest of all the cranial nerves, extend through the neck and cavity of the chest to the upper part of the abdomen. They supply nerves to the organs of voice and respiration, to the stomach aud the heart. The eleventh pair (244, M) are called the spinal accessory nerves, which receive their name from uniting with the trunk of the pnc-umogastric. They supply branches to the sternouiastoid and trapezius muscles. The last three pairs are classed by Willis as a single pair, and denomi- nated the eighth. The twelfth and last pair of cranial nerves are the hypoglossal (244, “'i. Their branches are spread over the fore part of the neck and the tongue. They act upon all the muscles connected with the hyoid bone, including th ■: ~e of the tongue, with the exception of the digastric, the mylo-byoid, and the middle constrictor of the pharynx. They are connected also with the pneumo- gastric and gustatory nerves, wdth several of the spinal nerves, and with those of the ganglionic system. 338. Spinal Cord. — As already explained, the continuation of the medulla oblongata after its passage into the vertebral canal, through the foramen magnum, receives the name of spinal cord ; which, divested of its sheaths and nervous appen- dages, is a rod of cerebral matter, descending through the canal to about two-thirds of its entire length, and terminating in a point. Its transverse section is generally circular, but, in cer- tain parts, slightly elliptical, the shorter axis of the ellipse being directed backwards and forwards, and the longer, right and left. A view of the entire length of this cord seen from behind is shown in figs. 247, 248, and 249 ; fig. 247 being the upper, fig. 248 the middle, and fig. 249 the lower third of its length. The first is therefore the cervical, the second the dorsal, and the third the lumbar part. It will be observed that its transverse diameter has an enlargement in the upper and lower parts, while it is uniform in the middle part. The transverse section is elliptical at the points of enlargement, and circular elsewhere. 339. The cord is traversed longitudinally by several fissures, which show that it consists, as is demonstrated by other means, of several distinct cords combined and adhering so as to form a single one. These two principal fissures traverse the middle of the anterior and pos- terior sides of the cord, the posterior fissure being shown in the figure at 1, 1, 1, 1, 1, 1. On either side of this is another fissure, marked l, -, which disappears about the termination of the first third of t e co . SPINAL CORD. 261 Two lateral fissures are shown at 7, 7, 7, 7, 7, 7, and two others similar to them have corresponding positions on the anterior side of the cord. It is in these fissures (7) that the roots of the nerves which issue on each side from the cord are implanted. 340. As has been just stated, the spinal cord consists of a Fig. 247.* Fig. 248.* Fig. 240.* POSTERIOR VIEW OF THE SPINAL CORD, DIVESTED OF ITS SHEATHS AND NERVOUS APPENDAGES AND CUT INTO THREE EQUAL SEGMENTS. combination of six lesser cords, which can be separated one from another, and are shown thus separated, but cut off a little below the summit, in fig. 250 ; where it will be observed that * From Ilirschfcld and Lcvoillu. 262 ANIMAL PHYSICS. the three cords of the right side intersect those of the left, before they unite in the medulla oblongata, or the pyramids, as they are called. It is only after thus intersecting and uniting that they ascend into the pons Varolii, and form the peduncles of the cerebrum, after which their constituent fibres diverge, and spread themselves out in all directions through the con- volutions of the lobes of the brain. Fig. 250.* VIEW OF THE CONSTITUENT CORDS OF THE SPINAL MARROW, SEPARATED, AND UNITING IN THE PYRAMIDS OF THE BRAIN. 0. Anterior cord dividing itself into two, of which the innermost contri- butes to the formation of the corresponding pyramid. 7. Middle or lateral cord, divided into four, which pass from the left to the right side, intersecting an equal number of similar ones coming from the opposite side, and taking the inverse direction. 8. The pyramids, of which the right is cut off immediately above the in- tersection, to show the olivary body (13) behind it. * From Hirsclifcld and Lovcilld. SPINAL CORD. 263 9. Left pyramidal bundle of fibres, traversing tbe pons Yarolii, and pro- ceeding to form the corresponding peduncle. 10. Section of the transverse superficial fibres of the pons. 11. Section of its deep fibres. 12. Left olivary body. 341. Spinal Nerves. — A series of nerves, called the spinal nerves, issue in pairs, one from each side of the spinal cord, each nerve having two roots ; one of which is implanted in the anterior, and the other in the posterior lateral fissure. These two roots, approaching each other, unite to form the nerve Avhich issues from the spinal cord at either side, passing, as already stated, through a lateral hole in the bony envelope provided for its exit. 342. A transverse section of the cord and its envelopes, made at a point where a pair of nerves issue from it, is shown in fig. 251, where 1 represents the dura mater ; 2 and 3 repre- sent the external and internal folds of the arachnoid, which is a double web, like a sheet of paper folded into two leaves, having a certain space between them. In the centre of the figure is represented a trans- verse section of the spinal cord having a deep median fissure on the posterior side, and a more shallow one on the anterior side. The substance of the cord consists of white and grey matter, but, contrarily to their arrangement in the brain, the white matter is exterior, and the grey interior. The quantity of the white greatly exceeds that of the grey. The form of the section of the grey matter is shown by the darkly shaded part of the figure. The space 7, between the internal fold of the arachnoid and the spinal cord, is filled with the cerebral fluid already described. The sheath formed by the continuation of the dura mater for the spinal nerve round the hole through which it issues, is shown at 9. 343. The posterior roots, proceeding from the posterior lateral fissure, in which they are implanted in the lateral groove are shown at 10, and the anterior, less numerous and smaller, proceeding from the anterior lateral fissure at 11. These two roots unite to form the nerve at the point of exit. On either side of the spinal cord is placed a ligament, indicated * From Hirschfcld and Levcille. 264 ANIMAL PHYSICS. in section at 12, called the denticulate ligament, which will presently be more fully explained. The manner in which the spinal nerves are connected with the cord may be illustrated also by the geometrical figures, figs. 252 and 253. In fig. 252, a front view of the spine is supposed to be presented, and a view of' the right side in fig. 253. The anterior median fissure is repre- sented at a a', and the posterior at p. The anterior lateral fissure is shown at c o', with the anterior root of the nerve lodged in it, and the posterior lateral fissure at b b', with the posterior root of the nerves also in it. The two roots unite at some distance from their insertion in the cord, and the posterior root, which is longer than the anterior, has an enlargement upon it, called its ganglion. The fibres, constituting the larger and posterior root, arc nerves of sensation, while those which constitute the lesser and anterior root are nerves of motion, as will be demonstrated hereafter. 344. Having thus described generally the cord and its appen- dages, their arrangement in the vertebral canal will be more clearly understood by reference to figs. 254, 255, 256, which represent a vertical section of the entire spinal cord, divided, as before, into three equal parts. The cord is supposed to be viewed from behind ; and the posterior roots of the nerves are shown on the right side, but are removed on the left, in order to render visible the form and points of attachment of the denticulate ligaments. The points of attachment of these ligaments arc indicated at 9, 9, 9 ; and it will be observed that they do not occur with regularity between nerve and nerve. The posterior roots of the right series of spinal nerves are shown at 10, 10, 10. The lateral fissure, in which the posterior roots of the left series of nerves (removed in the figure) are implanted, is shown at 1 1 , 11, 11. The commencement of the anterior roots Fig. 252. Fig. 253. SPINAL CORD. 265 of the left series of nerves is shown at 13, 13, 13. After issuing from the spine, each nerve divides, as shown at 14, into two branches, — a smaller posterior and a greater anterior branch, the latter being the continuation of the trunk. 345. The Spinal Cord, properly so called, terminates in a point, at 15, at a distance above the lower extremity of the vertebral column, equal to about a third of its length. From the lower part of the cord a bundle of nerves proceeds down- wards, lying in juxtaposition, and having a general resemblance to a horse’s tai^ from which this part has received the name of cauda equina. The nerves which form the cauda being those which, had the cord been continued downwards, would have * From the original of Hirsclifeld and Leveill<5. 266 ANIMAL PHYSICS. issued in pairs laterally, like those above them, still do so, as will appear by reference to the figure. Each pair of nerves, on arriving at the lateral hole which corresponds to them, turn at right angles, and issue horizontally or obliquely through the holes. To retain the spinal cord more steadily in the centre of the vertebral canal, a single ligamentous thread (16, 16), called the central ligament of the cord, is stretched from the point of the cord to the lowest point of the vertebral canaL 346. The series of nerves which thus issue laterally from the spine are grouped, by anatomists, in four classes, thus indi- cated in the figures 254, 255, and 256 : — I. to VIII. — Cervical nerves. I. to XII. — Dorsal nerves. I. to V. — Lumbar nerves. I. to V.- — Sacral nerves. 347. The nerves generally, whatever be their apparent origin, pass through the system in ramifications more or less complicated, and, like electric wires, only discharge their functions, whether of motion or sensibility, at their termina- tions. The nervous cords are thus subject to endless division and subdivision, until they become in many cases so infinitely minute as to escape all observation, even by the aid of the microscope. Since each fibre has its own peculiar destination and special function, and since this destination and function is in relation with the brain, it must be apparent that the various ramifications, in successively uniting together, as they approach their origin, can never be deprived of their proper functions, nor lose their individuality. It must not, consequently, be supposed that there is any analogy between the cases of blood- vessels running into each other, where the confluent streams are mixed, to form a single current after their union, and those of nerves coalescing, so that two or more fibres form a single cord. It must be considered, on the contrary, that in such coalition there is no actual mixture of nervous substance, and that the fibres are merely ranged side by side in mechanical juxtaposition, without any more intimate union. 348. These conclusions, which are derived from analogies of irresistible force, based upon the physiological properties of the nerves, are fully corroborated and confirmed by direct observa- tion. Each nervous cord is ascertained to be a bundle of fibres enclosed in a common sheath, these component fibres being very numerous, and of unequal thickness. In fig. 257 a nerve is represented, as drawn by Sir Charles SPINAL NERVES. 267 Bell, consisting of many cords, or funiculi, wrapped up in a common cellular sheath. a is the nerve, and b a single funiculus drawn out from the rest. Independently of the common sheath, or neurilemma, each particular component Fig. 257. cord has a sheath of its own. All these sheaths are composed of the same fibrous tissue, which appears to be nothing more than a continuation of the tissue which constitutes the neuri- lemma or sheath of the spinal cord. 349. The proper substance of the nervous filaments is an assemblage of flexible and hollow fibres of extreme tenuity, which are juxtaposed in parallel directions, having a milk- white colour, and throughout the whole length of the cord are so independent, that they can be isolated one from the other. Fontana, Remak, and Purkinje maintain that each of the constituent fibres of a nerve is cylindrical, and formed of two concentric tubes, one contained within the other. The central tube, consisting of a peculiar membrane, transparent and homogeneous, contains a whitish oleaginous humour, while the exterior tube is formed of a cellular substance. Ehrenberg and the most eminent micrographers maintain that there are two orders of primitive nervous tubes, which they denominate varicose and cylindrical. The varicose tubes consist of a series of alternate enlarge- ments and contractions, whence they are sometimes called articulated tubes, and contain a peculiar transparent liquid, which these physiologists call the nervous fluid. The diameter of these tubes varies from the 1200th to the 37000th of an inch. They are smaller as they pass from the centre to the external part of the brain, so that then.' nodosity is scarcely visible in the grey part of the cerebral matter. They are found more particularly in the nerves of special sensation, and in the medullo-encephalic axis. The cylindrical tubes are, as their name implies, uniform in diameter, and show no alternate enlargement and contraction like the former. They are filled with a white, viscid, and imper- 268 ANIMAL PHYSICS. fectly transparent liquid, which flows out readily in globule-. They are met with more especially in the nerves of the cerebro- spinal system ; in those of sensation as well as those of motion. According to Dr. Mandl, the fibres of the nerves of motion are much thicker than those of sensation. 350. Facial Nerves.— It has been already explained that, although the muscles are the proximate causes of motion in the animal organisation, they have not themselves any original moving power, and only become active under the stimulus of the nervous system. Wherever, therefore, there are muscles there must necessarily be corresponding nerves ; and the number and variety of these nerves, and their diffusion, will necessarily be proportionate to, and co-extensive with, the number, variety, and magnitude of the muscles. Although it would not be compatible with the purposes to which this volume is directed, nor the limits within which it must be circumscribed, to supply a statement and exposition of the nerves which throw the numerous muscles of the body into action, it will, nevertheless, be useful and interesting to convey some idea of their distribution throughout the system, and of the manner in which they ramify over all those parts to which motion is to be conveyed, or from which sensation is tc be transmitted. The nerves which overspread the superficial muscles of the head, face, and neck, shown in fig. 53, are represented in fig. 258. The principal of these, called the facial nerve, 25S, *, issues from an orifice under the ear called the stylo-mastoid foramen. It immediately gives off three branches, called the posterior auricular, 258, 2, the digastric, 258, ', and the stylo-hyoid, 258, s. The posterior auricular branch turns immediately backward beneath the ear, and is joined by the auricular branch of the cervical plexus, 258, 3. It throws out a branch, 258, 4, which supplies the occipital muscle ; another, 258, 5, which supplies the posterior auricular muscle : and a third, 258, 6, which supplies the superior auricular muscle. The trunk, 258, ’, proceeding forwards towards the jaw, divides into two primary branches, the ramifications of which spread over the side of the head, face, and neck. These two primary divisions are called the temporo-facial 258, 9, and the cervico-facial, 258, ls branches. The temporo-facial branch (9), which is the largest of the three, spreads its ramifications over the entire side of the face, extending as high as the temple and as low as the mouth, and from the ear to the eye. The cervico-facial branch (15) is first directed towards the angle of the lower jaw (16 and 17), whence it throws out numerous ramifications, which over- spread the muscles of the lower part of the cheek and chin, and others which descend towards the neck. Various other nervous trunks, variously denominated in anatomy from their local positions, may be seen to issue from foramina at 27, 25, 24, 22, 21, 19, 20, 28, and 18. Branches of FACIAL NERVES, 269 another nerve, called the cervical nerve, diverge in different directions from 32, some ascending to the ear and behind it, and anastomosing with Fig. 25S.* THE SUPERFICIAL SERVES OF TIIE FACE AND HEAD. the branches of the facial nerve ; others are thrown forwards to the front of the neck and the muscles under the jaw ; wlule others pass in the con- trary direction to the back of the neck. These nerves govern all the motions of the muscles of tha scalp, the ear, the mouth, lips, nose, and eyelids, the integu- ments of the ear, arid the upper part of the neck. * From Hirschfcld and Leveilll. 270 ANIMAL PHYSICS. The nerves here described, which are all ramifications of the seventh pan-, are exclusively motor, including no sensitive fibres. The parts to which they give motion receive sensibility from the nerves of the fifth pair, called the trifacial or trigeminal fig. 209." CERVICAL NERVES. * From Hirschfeld and Lcveilld. CERVICAL NERVES. 271 nerves. Thus the functions of motion and sensibility are in this case attached to different systems of nerves, while in the cases represented in the following figures of the cervical and other nerves, each cord is a compound one, which includes both motor and sensitive fibres ; and consequently while it governs the movements of the parts over which it is distributed, it also receives sensitive impressions from them, which it transmits to the nervous centre. 351. Cervical Nerves. — A system of nerves, also connected Fig. 200.* THE BRACHIAL PLEXUS. with the muscles of the neck and the lower part of the head * From the original of Hirschfeld and LevoillO. 272 ANIMAL PHYSICS. called the cervical plexus, is represented in fig. 259. A transverse branch (1) is directed forwards towards the jaw, and diverges into two ramifications, one (2) descending along the neck, and the other (3) ascending along the jaw. A branch (5), called the auricular, ascends to the ear. Various other branches (15, 16, 17, 19) descend to the chest. 352. Brachial Nerves. — The numerous muscles which, in layer over layer, invest the bones of the arm and hand, are moved, as may be expected, by a corresponding multiplicity of nerves which, issuing in a plexus, from the lower part of the neck (as shown in fig. 260) descend along the arm and hand, spreading in innumerable ramifications over each layer of muscles. Each nerve, entering the upper part of the arm from the neck, is a trunk which from one point to another in its descent throws out ramifica- tions ; and, in many cases, the branches proceeding from two or more of these trunks combine with each other. Some idea may be formed of this kind of complicated nervous ramifications by fig. 261, which represents the brachial plexus shown in fig. 260, the different nervous trunks which Fig. 261.* compose it having been separated in order to show the origin of each of the collateral and terminal branches. Thus it will he seen that the * From the original of Hirsclifeld and Levcilld. BRACHIAL NERVES. 273 branch at 7 is formed by the anastomosis of branches from the trunks 1, 2, and 3, the branch 11 by a combination of ramifications from 1 2, 3, and 4, and so on. The superficial nerves of the anterior part of the arm and hand are shown in figs. 262 and 263. Fig. 282.* Fig. 283.' The various ramifications which spread over the muscles of both parts of the arm and hand are rendered apparent in the figures, and eacli rami- fication has received special denominations from anatomists. The nerves which supply the deeper muscles of the arm and hand are shown in like manner in figs. 264 and 265. * From Hirschfeld and Levcillo. T 274 ANIMAL PHYSICS. Fig. 264.* Fig. 265.* 353. Crural Nerves. — The principal nerves which supply the superficial muscles of the thigh down to the knee are shown by a front Anew in fig. 266, and by a side view in fig. 267. The nerves in this, as well as in all the other figures, are represented as white cords, the veins being black, and the arteries being represented as a series of rings. The nerves which supply the superficial muscles of the leg From Hirschfeld and LcveilM. GANGLIONIC SYSTEM. 275 and foot are shown by a side view in fig. 268, and a back view in fig. 269. Fig. 260. » Fig. 267.* TIIE GANGLIONIC SYSTEM. 354. This system, also called the great sympathetic nerve consists of a series of small masses of nervous matter, called ganglions, t connected together by intermediate nervous cords in * From Hirschfcld and Lcvoillt-. t This is a Greek word, signifying tumour or enlargement. T 2 276 ANIMAL PHYSICS. such a manner as to form one continued chain communicating, in the one part by anastomoses with almost all the nerves Fig. 26S.* Fig. 2fi0.» of the cercbio-spinal system, and on the other spreading them- selves in innumerable fibres over all the organs of the involun- tary functions. 355. The Sympathetic Nerve, therefore, presides over the * From the original of Hirschfcid aud Leveiild. GANGLIONIC SYSTEM. 277 most important organic phenomena — the functions of the viscera, whose assemblage forms the apparatus of digestion, respiration, * circulation, and secretion, which are independent of the will or consciousness ; while, on the contrary, the province of the cerebro-spinal system is circumscribed by the play of the organs of sensation, perception, and motion. 356. The principal part of the ganglionic system is distri- buted symmetrically on either side of the median plane, imme- diately in front of the vertebral column. It extends upwards into the cranial, and downwards into the pelvic cavity, and presents numerous anastomoses along its course, as well as at its extremities. 357. A general view of the ganglionic system, showing its anastomoses with the spinal nerves, is represented in fig. 270. The chain of ganglions extending along the right side of the vertebral column, with their nerves anastomosing with the spinal, and spreading over the internal organs, will be recog- nised by the indications explained in the description of the figure. The superior part of the ganglionic system of the left side will also be seen by reference to fig. 271. EXPLANATION' OF FIGURE 270. The object of this figure is to display the vast assemblage of the parts of the ganglionic system, showing its connections with the principal cephalic nerves, such as the pneumogastric, trigeminal, &c., and with the spinal nerves. Preparation. — The anterior and lateral parts of the right side of the trunk, the corresponding portion of the base of the skull, the right branch of the lower jaw, and the zygomatic arch of the same side are removed. Several of the organs contained in the abdomen, the chest, the head, and the face have been cut away or drawn back so as to display the right ganglionic chain from the base of the skull to that of the coccyx. The connections of this chain, on the one hand, with all the spinal and some of the cerebro-spinal nerves, such as the trigeminal, the glosso- pharyngeal, the pneumogastric, the spinal accessory, and the hypoglossal ; and on the other hand, with all the ganglions and extra visceral plexus, have been preserved. ACCESSORY PARTS. a. Lacrymal gland. b. Sublingual gland. c. Submaxillary gland. d. Thyroid body. e. Trachea, of which the right bronchus is cut at its origin and slightly turned back, so as to show at the same timo its membranous and cartilaginous parts f. (Esophagus passing through the diaphragm. g. The stomach, hooked back to the left and cut open towards the pylorus so as to display the origin of the coronary stomachic plexus and the distribution of the two pneumogastric nerves. * Respiration is also under the influence of the cerebro-spinal system, as will presently be shown. ’ 278 ANIMAL PHYSICS. ^tJric^lexus^113* convo*u^ous sPread out so as to siiow the superior me-'.n- i. The transverse colon. j. The sigmoid flexure. k. The rectum. l. The bladder half inflated. m. The urethra. o. The seminal vesicle. p. The vas deferens. q. The spermatic cord, rr. The diaphragm. VASCULAR SYSTEM. A, The heart slightly drawn back to show the chief part of the cardiac and its secondary coronary plexus of nerves, right and left. B. The arch of the aorta hooked back, c. The brachio-cephalie trank. D. The subclavian artery, a part of which has been removed to disclose the inferior cervical ganglion. E. The inferior thyroid artery in connection with the middle cervical ganglion. !"■ Part of the external carotid artery, some branches of which have been pre- served to show the nervous plexus of the same name which interlace them. G. The internal carotid artery maintained in its channel and cut at its two ex- tremities. H. The thoracic aorta passing below the diaphragmatic opening. L The abdominal aorta. J. The common iliac artery. K. The intercostal vessels. l. The trunk of the pulmonary artery, the right branch of which is cut. M. The superior vena cava cut at its origin. n. The inferior vena cava. o. The pulmonary veins. CER EBRO-SPINAL SYSTEM. 1. The globe of the eye, from which a part of the sclerotica and cornea have been removed, to show the ciliary nerves, which, after having perforated the posterior part of the sclerotica, pass along the choroid and terminate in the ciliary ganglion. 2. Nerve of the inferior oblique muscle, from which issue parts of the motor root of the ophthalmic ganglion. 3. 3, 3. The three branches of the trigeminal nerve in connection with most of the cranial ganglions. 4. The ophthalmic ganglion. 5. The spheno-palatine ganglion. 6. The otic ganglion. 7. The submaxillary ganglion. S. The sublingual ganglion. 9. The external motor ocular nerve. 10. The facial, anastomosing with the otic and spheno-palatine ganglions. 11. The glosso-pharyngeal. 12. 12. The right pneumogastric. 13. The left pneumogastric spreading over the anterior surface of the stomach. 14. The spinal accessory. 15. The hypoglossal. 16. 16. The cervical plexus. 17. The brachial plexus. 18. IS. Intercostal nerves. 19, 19. Lumbar plexus. 20. Sacral plexus. GANGLIONIC SYSTEM. 21. Superior cervical ganglion. It gives origin above to the two carotid branches which form the carotid plexus arouud the carotid artery, from which emanate or terminate anastomoses with the following nerves : — 22, The norve of Jacobson, n.u as c GENERAL VIEW OF TOE CANOLIONIC SYSTEM, Showing its connection with the Cranial and Spina. Nerves, reproduced from the original, drawn by M . LeveilltS, from a preparation made oy M. Hirschfold, witn the permission of the authors aud of M. J. B. Baillifere, tho publisher. [To face page 273.1 GANGLIONIC SYSTEM. 279 23. The carotid filament of the Vidian nerve. 24. The external ocular motor. 25. The ophthalmic ganglion. 26. Filament for the pituitary gland. 27. Anastomoses of the superior cervical ganglion with the first cervical pair. 2S. The pharyngeal and carotid branches. 29. The pharyngeal and intercarotid plexus. From the latter emanate secondary plexus, which interlace all the branches of the external carotid, as may be seen in the figure in the case of the facial and lingual arteries. 30. Laryngeal branch connected with the external laryngeal of the pneumo- gastric, to form the laryngeal plexus of Haller. 31. The superior cardiac nerve. 32. Cords connecting the superior cervical ganglion with 33. The middle cervical ganglion, among the internal branches of which may be 34. 35. 36. 37. 38. 40. 41. 42. 43. 45. 47. 4S. 49. 50. 51. 52. 53. 54. 56. 58 64 observed The anastomosis, with The recurrent nerve, The middle cardiac nerve, and several filaments which interlace the inferior thyroid artery, a branch of the subclavicular artery. The external branches of the middle cervical ganglion throw themselves into the brachial plexus. Cord joining this ganglion with The inferior cervical ganglion. Filaments of the latter around the subclavian and vertebral arteries. Branch anastomosing with the first intercostal nerve, Cardiac plexus and ganglion. 44. Secondary plexus of the right and left coronary arteries. 46. Thoracic ganglionic chain. It is connected without with the inter- costal nerves. Within, the first five ganglions furnish numerous very delicate filaments, of which some are thrown into the cardiac plexus, others into the pneumogastric, and others upon the aorta and into the periosteum of the vertebrae. The five lower ganglions give internal branches, which anastomose with each other, and form The great splanchnic, which passes through the diaphragm, to find its way to The corresponding semilunar ganglion. The little splanchnic formed by one or two branches issuing from the last two thoracic ganglions. This nerve presents here a slight enlargement assisting in the formation of the renal and solar plexus. Solar plexus formed by a mass of ganglions, and by the interlacing of large intermediate branches with the right and left semilunar ganglions. This plexus receives The anastomoses of the pneumogastric, and Of the phrenic nerve, which presents at this place the diaphragmatic ganglion. The solar plexus furnishes the following secondary plexus : — The stomachic coronary. The hepatic, the splenic, and The superior mesenteric. These four great plexus, as well as all the other visceral plexus, interlace the arteries whose names they bear. The renal plexus. to 58. Lumbar ganglionic chain. It anastomoses without, with the lumbar nerves, and furnishes within, several branches, which, aftor having anasto- mosed with those of the opposite side, and with a considerable prolongation of the solar plexus, form . The bimbo-aortic plexus. This last presents generally two flat enlarge- ments, one (60) above, and the other (61) below the bifurcation of the aorta. It supplies (62) the spermatic, and (63) the inferior mesenteric plexus. Below its inferior enlargement, the lumbo-aortic plexus bifurcates, embracing the rectum, and throws itself into the hypogastric plexus. The hypogastric plexus, formed by the preceding bifurcation, by the viscoral branches of tho sacral plexus, by several branches proceeding from the sacral and lumbar ganglions, and, in fine, by tho termination of tho iuforior mesenteric plexus. It produces tho vesical, prostatic, and sponnatic plexus and concurs with other nerves proceeding from the renal and lumbo-aortic plexus, in the formation of tho plexus of the spermatic cord. The sacral ganglionic chain. It anastomoses without with the sacral plexus Within, its branches anastomose with each other, and with those of tho opposite side, and throw themselves into the hypogastric plexus. The ganglionic system terminates with the coccygeal ganglion, at the' iowost GENERAL VIEW OF THE PRINCIPAL NERVES OF TUE NECK. 1. Pneumogastric nerve, or. tenth cerebral ; the principal branches of which anastomose with the filaments of the great sympathetic, and spread over the lungs and stomach. 0, 7. Branches of the pueumogastric passing to the larynx. 0, 9. Recurrent nerve, being a branch of the pueumogastric, which is bent upwards under the aorta, from the top of the thorax to the larynx. 280 ANIMAL PHYSICS. Fig. 271.* Reproduced from the figures of Messrs. Ilirschfeld and Leveilld. GANGLIONIC SYSTEM. 281 10, 11. Cardiac branches passing to the heart. 13. Pulmonary plexus. 14. Lingual nerve. 15. Terminal part of the great hypoglossal nerve. 16. Glosso-pharyngeal nerve. 17. Spinal accessory or eleventh cerebral nerve. IS. Second cervical nerve. 19. Third cervical nerve. 23. Sixth, seventh, and eighth cervical nerves, joining with the first dorsal nerve to form the brachial plexus. 24. Superior cervical ganglion of the great sympathetic. 25. Middle cervical ganglion of same. 26. Inferior cervical ganglion of same. 27. 28, 29, 30. Dorsal ganglions. 358. The origin of the ganglionic system has been long disputed, and is still a question upon which physiologists are not agreed. According to Winslow and Reil, who have been followed by Bichat, the ganglionic is regarded as a special nervous system. The ganglions which compose it are so many small centres of nervous influence independent of each other and of the cerebro-spinal system, and communicating with each other and with the latter by intermediate branches. Sarlandiere and Burdach consider the ganglionic system as deriving its origin from the internal organs, and terminating in all the points of the cerebro-spinal system. This view, though not much favoured by physiologists, is countenanced by the early development of the great sympathetic, which is anterior to that of the other parts of the nervous system, and by its existence in acephala and other species which are altogether deprived of the cerebro-spinal axis. Other anatomists, ancient and modem, maintain an opinion diametrically opposed to this, which regards the great sympa- thetic as emerging by numerous roots from the cerebro-spinal axis, and, after undergoing remarkable modifications in passing through the ganglions, terminating in the internal organs. This is the doctrine most generally received. NERVOUS SYSTEMS OF INFERIOR ANIMALS. 359. Independently of the interest which must attach to the investigation of the varying structure and distribution of the nervous system in the lower species, the comparative anatomy of the encephalon and its appendages derives great importance from the instrument into which it has been converted with such signal success in determining the functions of the several parts of the system. Without a general knowledge of the compara- tive anatomy of the brain, such observations would supply no 282 ANIMAL PHYSICS. conclusive result. It is, therefore, important here, before noticing the discoveries which have been made respecting the functions of the nervous system, briefly to explain the principal differences which are observed in the cerebral organs of the inferior animals compared with man, and with each other. 360. The nervous system of mammifers, birds, reptiles, and fishes, is constructed upon the same general plan, and consists of the same chief parts as have been described in the case of the human race. In all there is a cerebro-spinal axis, including cerebrum, cerebellum, medulla oblongata, and spinal cord, with their appendages and divergent nervous trunks. In all there is also a ganglionic system ; and the nerves which issue from the former have the properties characteristic of animal life, while those which issue from the latter have the properties of organic life, as in the nervous system of the human organism. 361. ^Nevertheless, there are considerable differences, as well between the human encephalon and that of inferior classes, as between those of inferior classes compared one with another. Even upon a superficial view of the human encephalon com- pared with those of inferior animals, it will appear that the cerebral hemispheres are comparatively more extensive, that their convolutions are in general more numerous and compli- cated— presenting, consequently, a proportionately greater ex- tent of surface ; and, in fine, that while in man they completely overlap the cerebellum and all subordinate parts projecting more or less beyond them, they leave, in all inferior species, a portion of this part of the encephalon uncovered, and the portion so exposed is greater and greater as we descend in the scale of organisation. When the human encephalon is viewed from above, nothing is visible between the extreme limits before and behind save the convolutions of the cerebrum (fig. 240). If it be viewed upon its base (fig. 244), the cerebrum is seen (244, *) projecting sensibly beyond the extreme limits of the cerebellum. 362. Quadrumana. — In the quadrumana, the class which bears the closest analogy to the human race, the encephalon viewed from above (fig. 272), and from below (fig. 273), presents the same general aspect as the human encephalon ; but the cerebellum is seen, in fig. 272, from above projecting beyond the hemispheres of the cerebrum. The convolutions also are less numerous and complicated ; wliile in the case of the human encephalon viewed upon its inferior surface (fig. 244), the BRAIN OF QUADRUMANA 283 cerebrum appears beyond tbe cerebellum, no such appearance is Fig. 272.* ENCEPHALON OF THE OHANG-OUTANG. Fig. 273.* Fig. 274. f ENCEPHALON OF THE LION SEDUCED. observed on the base (fig. 273) of the encephalon of the quadrumana. Trcviranus. t Leuret. 2S4 ANIMAL PHYSICS. ^oG3. Carnivora. — In the case of the carnivora (figs. 274, 2j5, 27 G, 277, and 278), the encephalon viewed from above discloses a still greater portion of the cerebellum and the medulla oblongata. Fig. 27o.' ENCEPHALON OF THE SEAL. Fig. 276.* Fig. 277.* Fig. 27S.* ENCEPHALON OF THE HEDGEHOO. ENCEPHALON OF THE ENCEPHALON OF THE MOLE. BAT. 364. Marsupialia. — In the case of the marsupiata (fig. 279), * Leurct. BRAIN OP CARNIVORA AND MARSUPIATA. 285 the disclosure not only of the cerebellum is complete, but also of the parts above it. Fig. 279. ‘ ENCEPHALON OF THE OPOSSUM. Fig. 280.* ENCEPHALON OF TnE BEAVER REDUCED. Fig. 281.* ENCEPHALON OF THE GIRAFFE REDUCED. Owen. 286 ANIMAL PHYSICS. 365. Rodents— In the rodents (fig. 280), the cerebellum i- half exposed, and in the ruminants more than half exposed, (fig- 281). It may be observed in general, that, in proportion as the cerebral hemispheres decrease in size, the subordinate parts, such as the corpora quadxigemina, are relatively enlarged. 366. Birds. — In birds (fig. 282) the cerebrum is so far de- creased as to disclose not only the cerebellum, but also the corpora quadrigemina.* The encephalon of a common domestic fowl is shown by its superior surface in fig. 282, by its inferior surface in fig. 283, and by a side view in fig. 284. Fig. 282. (R. An.) Superior Surface. Fig. 2S3. (R. An.) Base. Fig. 2S4. (R. An.) Left side-view. ENCEPHALON OF A FOWL. r roper] y these parts of the brain in birds and inferior species are called bigemina, two only being found instead of four. I BRAIN OP REPTILES. 287 The following axe the parts designated in this as well as all the succeeding figures : — a. The spinal cord. a1. The medulla oblongata. b. The cerebellum. c. The quadrigeminal tubercles. d. The cerebral hemisphere. c. The pineal gland. /. The optic thalamus, on which the pituitary gland is'implanted, and which appears only in the side views. 1. Olfactory nerves. 2. Optic nerves. 3. The third pair, or motor nerves of the eye. 4. The fourth pair, or pathetic nerve. 5. The fifth pair, or trifacial nerve. 6. The sixth pair, or abducent nerve. 7. The acoustic nerve. 8. The pneumogastric nerve. 9. The glosso-pharyngeal nerve. 0, 0, 0. Spinal nerves. 367. Reptiles and Amphibia. — In reptiles, fig. 285, fig. 286, and fig. 288; and in amphibia, fig. 291, the cerebral hemispheres Fig. 285.* Superior side. Fig. 2S6.* 13asc. ENCEPHALON OF THE CnOCODILE (CrOCodilus lucius). are still further diminished, and the other parts more complete exposed. 1 Cuvier. 288 ANIMAL PHYSICS. Fig. 287.* Left side-view. ENCEPHALON OP CROCODILE (Crocodilus lucius). Superior surface. Base. Leftside-view the adder (Coluber liatrix). Fig. 291.* Superior surface. Fig. 292.* Base. Fig. 293* Left side-view. encephalon or the common FRoo (Rana csculenta). 368. Fishes. — Tho encephalon of fishes, fig. 294, consists of a series of single and double enlargements, the character of * Cuvier. BRAIN OP FISHES. 289 which is indicated by the letters and numbers, as in all the former figures. The posterior enlargement, which is single, is the cerebellum : in front of this are the two quadrigeminal bodies, which in this class constitute the largest part of the encephalon. They are hollow in their interior, and include the origin of the optic nerves. The two lesser bodies in front of these are the analogues of the cerebral hemispheres. In comparing the brain of reptiles and birds with that of mammifers, it is found that the corpus callosum is incompletely formed. The quadrigeminal bodies are hollow, the corpora albicantia are absent, some deep fibres only of the pons Varolii are present ; and the lateral parts of the cerebellum are much less developed. 369. The central part of the nervous system of the various classes of animals, such as mollusca, insects, and Crustacea, which have no vertebral column, differs altogether in its form from that of the higher classes. In the latter, the encephalic mass is separated from the oesophagus, lying altogether above or behind it. In the former, on the contrary, it is collected round the oesophagus, forming a ring of nervous matter, which on the dorsal side presents a large ganglion, the analogue of the brain. There is another enlargement, or ganglion, on the lower part of the ring, from which the rest of the nervous system springs : that system consisting generally either of single nervous cords, or of a series of ganglions similar in form and Fig. 294.* Superior surface. Fig. 295.* Base. Fig. 296.* Left side-view. ENCEPHALON OF THE COMMON PERCH. * Cuvier. U 290 ANIMAL PHYSICS. arrangement to those of the great sympathetic nerve in the human organism. 370. Invertebrata.— Physiologists have been much divided in opinion as to the character of the nervous system of the invertebrata. Ackermann, Reil, Bichat, and more recently Serres and Desmoullins, considered it to belong to the same class as the sympathetic nerve of the higher animals. On the other hand, Scarpa, Blumenbach, Cuvier, Gall, and J. F. Meckel have rejected all such analogy, and regarded the abdo- minal cord or chain as representing the spinal cord of the vertebrata. Meckel and P. H. Yon Walther considered that the nervous chain of the invertebrata, extending from the brain into the trunk of the body, was endowed with the properties of both the cerebro-spinal axis and the ganglionic system ; but that in the mollusca it partook more of the character of the latter, and in the articulata more of that of the former. Muller considers, in fine, that this question has been settled by the discovery arising from the researches of Grant, Newport, Treviranus, and himself, that in most articu- late animals, and in all insects, there is — besides the abdo- minal cord representing- the cerebro-spinal system — another system of nerves, consisting of delicate and minute ganglions, destined solely for the viscera, and analogous therefore to the ganglionic system of the superior animals. In short, there appears to be no class of animals, however low in the scale of organisation, in which some traces of a nervous system are not discoverable. Ehrenberg has detected them even in the infu- soria, and has distinctly seen them in the rotifera. 371. Radiata. — The most simple form of nervous system, the existence of which has been determined in the invertebrata. consists of a mere ring of nervous matter surrounding the oeso- phagus. In the radiata, fig. 297, this ring is destitute of any ganglionic enlargements. Its ramifications are distributed according to the radiated structure of the animaL The figure represents the nervous system of the asterias, or starfish ; 1, being the feet ; 2, the feet cut off transversely ; 3, the openings through which the feet projected ; and 4, the nervous ring surrounding the mouth, giving off three branches to each raj-. The type of the structure of this class being the repetition of similar organs disposed concentrically, the nerves which issue from the central ring NERVOUS SYSTEM OF MOLLUSCA. 291 Lave a corresponding disposition. These radiating nervous branches, of Fig. 297.* NERVOUS SYSTEM OF THE STARFISH. which none has more importance than another, constitute collectively the analogue of the cerebro-spinal axis in the higher animals. 372. Mollusca. — Ascending from the radiata to the mollusca, a spinal cord, with its ramifications, begins to make its appearance. The body of this class consists of a convolution of viscera, the sensitive and motor functions of which appear to be limited to a dull tactile sense and sluggish power of locomotion. There are, nevertheless, three sets of nerves analogous to the nerves of sensation, those of motion, and the sympathetic nerves of the higher animals. The viscera and locomotive organs having no symmetry of arrangement, no corresponding order prevails in the nervous system, the type of which is a ring surrounding the oesophagus, from which chains of ganglia are given off in different directions, ac- cording to the disposition of the organs. Fig. 297.* NERVOUS SYSTEM OF THE BLACK SLUG. u 2 MUller, after Tiedcmumi. 292 ANIMAL PHYSICS. As an example of this class, fig. 298 represents the nervous system of the common black slug, Limax ater. 1, 2, and 3 represent ganglions, the first above, the second below, and the third before and under the oesophagus. The third are connected by a long delicate filament with the first, and by a transverse filament with each other. They also send branches forward to the mouth and backward along the oesophagus to the stomach. 4, 4, are the optic nerves ; 5, those of the upper lip ; 6, those of the tentacula and the mouth ; 7, those of the respiratory organs ; 8, those of the genital sac, &c. ; and 9, the large nerves of the mouth. 373. Insects. — In passing to the insects and articulata in general, the nervous system varies its form, accommodating Fig 299.* Fig. 300.* NERVOUS SYSTEM OF THE SPHYNX LIGUSTBI. itself to the varying form of the structure of the body, and assuming by slow degrees features more and more resembling those of the nervous system of the higher animals. The general character of the articulata consists of a succession of rings similarly organised, jointed together, each ring containing After Mttller. NERVOUS SYSTEM OP INSECTS. 293 similar parts of the vascular system. In accordance with tMs structure, the ganglions proceeding from the ring of nervous matter surrounding the oesophagus are formed into a chain, each ganglion throwing out corresponding systems of nerves. According to Muller, the brain in all the species of annelides, insects, arachnides, and crustaceans, is above the oesophagus ; and in insects, the nervous system of the viscera, correspond- ing in its functions to the sympathetic nerve, is distinctly developed. In figs. 299 and 300, the nervous system of the Sphynx ligustri, a species of hawk-moth, is presented ; in the former as it exists in the larva, and in the latter as in the perfect insect. The brain, consisting of a ganglion above the oesophagus, is at a, and 1, 2, 3 to 12 are the succes- sive ganglia of the abdominal cord, each throwing off laterally like systems of nerves. The median visceral nerve (b) arises by two roots from a. Two nerves also arise from a, on each side forming a small ganglion ; these likewise belong to the visceral or sympathetic system. Interganglionic, or transverse nerves, are shown at c. cl are the nerves given off by the ganglia. In comparing the system of the larva with that of the perfect insect, it will be seen that some of the ganglia coalesce during the metamorphoses, so as to form larger masses in accordance with the altered disposition of the parts. The nervous systems of several other classes of insects are represented in figs. 301 to 305. Fig. sol. A SPECIES OF FIELD BEETLE. 294 ANIMAL PHYSICS. Fig. 302. Fig. 303. Fig. 304. Fig. 305. EARWIG. GRASSHOPPER. STAG-BEETLE. PIELD-BCG. In the last figure b and c are the optic nerves, d the thoracic, and e the abdominal ganglions. FUNCTIONS OF THE NERVOUS SYSTEM. 374, When the structure of the cerebro-spinal axis, and more especially that of the encephalon, is considered, it is impossible to resist the persuasion that these curiously formed and beautifully symmetrical parts exercise severally distinct influences in the economy. That varieties of structure so remai'kable must be attended with corresponding varieties of function, is a proposition the truth of which is all but intuitive. The convoluted and vermiform cerebrum (fig. 244, 19), consisting of white matter with a grey cortical coating ; and the laminated and foliated cerebellum (fig. 244, 17), consisting of grey matter with some intermixture of white, and exhibiting within that remark- able arborescence already indicated (fig. 240/), must obviously be endowed with different nervous powers. The pons Yarolii FUNCTIONS OF THE NERVOUS SYSTEM. 295 (fig. 244, 1S), tied by its crura aud peduncles to the cerebrum, the cerebellum aud the medulla oblongata, with its white fibres, transverse and longitudinal, interspersed with grey matter, — the quadrigeminal tubercles (fig. 245, ae), ranged two on each side of the median plane and one before the other, consisting, con- trary to the structure of the cerebrum, of white matter without and grey within, tied by white cords to the cerebellum and the optic thalami, — the corpus callosum (fig. 240, 26), traversing the longitudinal fissure, and connecting the two cerebral hemi- spheres,— the medulla oblongata (fig. 244, 13), with its pyramids and rope-like (restiform) cords, its olive-formed masses (olivary bodies), decussating pyramids, and curiously disposed white and grey matter,— in fine, the multiple spinal cord (figs. 247 to 256), with its fissures and furrows, and its singularly formed pith of grey, surrounded by the larger mass of white fibrous matter — severally present varieties of structure which cannot be admitted to have been contrived and produced without some intelligent purpose. It might therefore be expected that their respective functions would be assigned to these parts, and connected with them by evidence based upon sufficient induction by the researches of physiologists. Such an investigation, nevertheless, presented difficulties which, until a very recent period, rendered all the labours of experimental inquirers and observers fruitless. In the problems commonly presented to the experimental philo- sopher, the phenomena to be compared and brought into the relation of cause and effect are all physical, and capable of immediate observation, either when naturally evolved, or when provoked by the express contrivances of the observer. In the present case, however, the phenomena to be compared are mental on the one hand and physical on the other. A physical effect is to be traced to a mental cause, or a mental effect to a physical cause. The physical effects being organic, and connected with and dependent on vital influences, are extremely difficult of obser- vation ; and the mental phenomena with which they stand in relation, much more so. Beset with such difficulties, the extent of the discoveries actually made by physiologists respecting the functions of the nervous system, is a legitimate subject of admiration, although the field which still remains to reward the labours of future observers be still more considerable. 375. Sensibility.— The impressions produced by external 296 ANIMAL PHYSICS. physical agents upon the organs are received by the terminal parts of the nervous ramifications diffused through these organs, and by a peculiar property of the nerves they are propagated thence to the seat of perception, which, as will be shown hereafter, is the cerebral hemispheres. This property of the nervous fibres to receive and transmit these impressions is called sensibility, and the impressions so transmitted are called sensations. 3 7 6. Perceptibility. — The faculty of the cerebrum to receive the impressions thus transmitted is called perceptibility, and the mental consciousness of the impression so received is called perception. 377'. Volition.— But the cerebral hemispheres, as will hereafter appear, are also the seat of will , and the nervous fibres are endowed with another faculty, in virtue of which they are susceptible of receiving definite impressions from the mental act of the will, of propagating such impressions to the organ or member indi- cated by the will, and of imparting to such organ or member, through the agency of the contractile power of the corresponding muscles, the particular motion prescribed by the will. 378. Excitability — This faculty of the nervous fibres, in virtue of which they are susceptible of receiving impressions from the will, and of propagating these impressions to the part indicated, is called excitability, and the state of the nervous fibre when so acted upon is called excitation. 379. It will presently be demonstrated that the nerves endowed with sensibility are different from, and independent of those . endowed with excitability ; the former being distin- guished as nerves of sensation, and the latter as nerves of motion. In general, however, the same nervous cord includes fibres of both kinds, which, though placed in mechanical juxtaposition, have no reciprocal physiological influence what- soever. 380. Senses. — The senses, usually enumerated as five, con- sist of two classes, special and general. The special senses are those which are susceptible of impressions only by particular physical agencies, and are those of seeing, hearing, smelling, and tasting — the first being susceptible of no impressions but those of light ; the second, of none but those of sound; the FUNCTIONS OF THE NERVES. 297 third, of none but those of odorous effluvia ; and the fourth only of those produced by the sapid particles of bodies. 381. Tactile Sense. — The general sense is that of touch or feeling, called the tactile sense, which receives its denomination from its susceptibility of receiving impressions from bodies, in whatever form or state they may exist. By this sense we perceive the qualities of shape, weight, texture, superficial conformation, softness or hardness, and temperature. When any part of the external surface of the organs is brought into contact with a body, we immediately become conscious of the qualities just indicated; we feel that it is smooth or rough, soft or hard, sharp, blunt, or round, and warm or cold. These various impressions are received by the fibres of the nerves of sensation which are diffused over the surface of the organ which is in contact with the object, and they are propagated by these nerves to the cerebrum, where the sensation is perceived. 382. It may be asked, whether impressions so very different as those of temperature and texture are received and propa- gated to the sensorium by the same nervous fibres ? This is a question to which physiologists have not given a solution. If the different impressions of the tactile sense are received by different nerves, the fact has not yet been ascertained by expe- riment or observation. The nerves, therefore, of this sense, must for the present be regarded as being susceptible of receiving and transmitting at one and the same moment dis- tinct and independent impressions of all the different physical qualities above mentioned. 383. The functions of the nervous system, so far as they are known, have been ascertained and demonstrated, partly by observations made upon the human subject when placed under the exceptional conditions incidental to disease, to surgical operations, and post-mortem experiments, but chiefly by expe- riments made upon inferior animals, placed under conditions expressly contrived to provoke such manifestations as might enable the observer to connect, in the relation of cause and effect, the mental and organic phenomena. The result of such researches, combined with the known and admitted analogies of structure and function prevailing between the human and animal organisms, have led to the most important discoveries in the physiology of the nerves. If the nerve which connects any member of an animal with the cerebro-spinal axis be denuded, as it may be, at any point 298 ANIMAL PHYSICS. of its course, by the dissection of the surrounding parts, and be submitted to mechanical irritation, two effects will immediately ensue : the animal will exhibit unequivocal manifestations of pain ; and the member with which the nerve is connected will be agitated with convulsive motions. 384. In this case, the nervous cord submitted to irritation includes both fibres of sensation and fibres of motion. The irritant applied to the cord acts upon the excitability of one set of fibres and the sensibility of the other. The excitation pro- duced in the fibres of motion descending to the extremities of the nerve, diffused over the muscles of the member, excites their contractility, and produces the convulsive motion. The same irritating agent, acting on the fibres of sensation, excites their sensibility ; and the sensation thus produced is propagated upwards to the cerebrum, where it causes the perception of pain. In this case, therefore, the irritating agent, applied to the neivous cord, plays at once the part of the will in producing the excitation of the nerves of motion, and of the tactile sense in exciting the nerves of sensation. The excitation of the former descends from the point irritated to the member moved, and the impression on the latter ascends from the same point to the nervous centre. 385. If a ligature be placed on the nervous cord, it will interrupt the propagation both of excitability and sensibility. If, in such case, the nerve be irritated below the ligature, the excitation will descend to the member, and produce, as before, the convulsive motion ; but no pain whatever will be manifested, showing that the transmission of the impression upwards on the nerves of sensation is interrupted by the ligature. If the nerve be irritated above the ligature, the propagation of the excitation downwards on the nerves of motion will be stopped, as will appear by the absence of all convulsive motion in the member ; but the same manifestation of pain will take place as was exhibited before the ligature was applied. 386. If two ligatures be applied at two distant points of the same nervous cord, and, at the same time, all the ramifications issuing from the cord between the ligatures be cut off, a series of phenomena may be produced which will further illustrate these functions of the tactile and motor nerves. In this case, if the nervous cord be irritated between the ligatures, neither convulsive motions nor pain will be produced, the lower ligature FUNCTIONS OF THE NERVES. 299 intercepting the one, and the upper ligature the other. If the upper ligature be then removed, pain will be manifested, but no motion produced. If the lower ligature be removed, main- taining the upper one, convulsive motions will be produced, but no pain manifested. If both ligatures be removed, convulsive motions will be produced, and pain will be manifested. If, in line, after the ligatures have been applied, and before the intermediate ramifications have been cut, irritation be applied beyond the lower ligature, convulsive motions will be produced in those parts to which the ramifications between the lower ligature and the point of irritation extend, but no pain will be manifested. 38 T. In general, when irritation is applied to any point of a nervous cord without ligatures, convulsive motions will be pro- duced in all the parts to which the ramifications below the irritated point extend, but in none of the parts above it ; and the sensation, which is propagated upwards to the sensorium, will not diverge into the ramifications of the cord placed above the irritated point. The perception of the sensation, in such ■case, will be the same as if the parts to which the ramifications ■of the nerves of sensation below the irritated point extend, had been the seat of the pain. 388. The various phenomena above described have been ■developed in experiments made on the inferior animals, by several physiologists, but more especially, and on a large scale, by M. Flourens. That eminent physiologist, among other experiments to the same effect, denuded the sciatic nerve of a dog by an incision extending from the great trochanter to the ham, and intercepting a part of it between two ligatures, per- formed all the experiments above described. When the ligature above the point irritated was applied, the annual showed no signs of pain ; but when it was removed, the animal yelled loudly, and struggled to escape. In the same manner, so long as the inferior ligature was maintained, the leg remained immovable, but when it was removed, violent convulsive motions were produced. Like results were obtained in more than twenty different experiments on different animals. 389. If, instead of applying a ligature as described in the above experiments, the nervous cord be divided at a given point, like effects will ensue. Irritation applied at the stump of the lower branch will produce convulsivo motions in the parts 300 ANIMAL PHYSICS. to which the ramifications of the branch extend, provided that the irritation be applied soon after the nervous cord is divided ; for, in a certain period after its separation from the cerebro-spinal axis, it will lose its excitability. Irritation applied to the stump of the upper branch will produce the sensation of pain, and the perception of such sensation will be the same as if the seat of pain were the parts to which the ramifications of the lower cord cut off extend. This is a pheno- menon familiar to all operating surgeons, and it is a physiological illusion which, in most cases, continues through life. 'When a limb has been amputated, the stumps of the nerves which ramified through the part cut off maintain their sensibility ; and the effect produced upon the individual, thus mutilated, is the same as if he still retained the amputated member, and felt all the sensations in it. The sensations are most vivid, while the surface of the stump and the divided nerves are the seat of inflammation, and the patient then complains of severe pain felt, as if in the whole limb which has been removed. When the stump is healed, the sensations which we are accustomed to have in a sound limb are still felt ; and frequently throughout life there is a tingling, and often pain felt, which are referred to the parts that are lost. These sensations are not of an undefined character ; the pains and tingling are distinctly referred to single toes, to the sole of the foot, to the dorsum of the foot, to the skin, etc. It is ridiculous to attribute these important phenomena to the action Of the imagination, represents the arteries, and a' the veins ; and as the blood which flows through n is nutritive or red blood, and that which flows through a' is vitiated or black blood, the former is generally called arterial, and the latter venous blood. It must be observed, however, that in the pulmonary system of blood- vessels the character of the blood which they conduct is reversed ; the pulmonary arteries are represented by d', con- ducting vitiated or black blood, while the pulmonary veins, represented by a, conduct nutritive or red blood. 453. Lymphatics.— The single tube, indicated at F, is one which, in the actual organisation of the body, diverges and ramifies, like the veins and arteries, into innumerable small tubes, which penetrate all parts of the system. These receive z 338 ANIMAL PHYSICS. the lymph and chyle, by a process called absorption, from whence these vessels were called absorbents. They are now more generally denominated lymphatics. The single tube f, into which these numerous lymphatics finally coalesce, and through which their contents are discharged into the vein which enters the right auricle of the heart, is called the thoracic duct. The liquid discharged into the veins as they approach the heart, has been called from its colour white blood. Of its components, the lymph is a colourless and transparent liquid, like water, while the chyle is an opaque white liquid, like milk. Hence, those lymphatic tubes which conduct chyle chiefly or solely, have been sometimes called lacteals. 454. Internal Structure of the Heart. — In fig. 311 is repre- sented a theoretical section of the heart, supposed to be made by a vertical plane, parallel to the breast and back, and to be viewed from the front ; the right side of the figure being consequently the left of the heart, and vice versa. It will be perceived that the internal structure, being hollow, is divided into four chambers by one vertical partition, and by two which are nearly horizontal, but a little inclined one to the other, so as to form a very obtuse angle. "While the vertical partition is an absolute wall, dividing the heart into two independent compartments, the two horizontal partitions have valves, both of which open downwards, so that while any pressure from above will open them, making a free communication between the upper and lower compartments, any pressure from below will close them, so as to render the two compartments separate. The names of the principal parts are so indicated on the figure, that it woidd be superfluous, after what has been already explained, to elucidate them further. It may, however, be observed, that both the amides and the ventricles are constituted by powerful muscles, by the alternate contraction and relaxation of which the blood, with which the several compartments are filled, is propelled from the auricles to the ventricles, from the ventricles to the arteries, and thence through the other parts of the syrstem. 455. Course of the Blood. — The embouchure of the aorta being placed at the upper and inner corner of the left ventricle, the tube passing upwards is bent into the form of a shepherd s crook, and passes downwards behind the heart, and between STRUCTURE OF TIIE HEART. 339 it and the spinal column. The two pulmonary veins which proceed from each lung, right and left, coalescing before arriving at the heart, have their embouchures at the internal and external comers of the left auricle. The blood flowing in through these, fills that auricle, and, passing then through the mitral valve into the left ventricle, issues thence through the aorta, and is distributed through the arteries. 456. Valves of the Heart. — The blood which returns through the veins arrives at the right auricle by two large veins, called the upper and lower hollow veins, as will be seen in the figure. It issues from the right auricle through the valve between that and the right ventricle, called the tricuspid valve, and from the right ventricle it passes, at the upper and inner corner, into the common trank of the right and left pulmonary arteries, which divei'ge one from the other immediately under the bend of the aorta. A orta. Right pulmonary artery. ] Left pulmonary artery. Upper vena cava. Right pulmonary veins Right auricle — Tricuspid valve - - Lower vena cava — Right ventricle Vertical partition Aorta ( Left pulmonary i veins. Left auricle. Mitral valve. Left ventricle. Fig. 311. THEORETICAL SECTION OP THE HEART, SCPPOSED^TO BE SEEK FROM THE FRONT. 457. Position of Heart and Lungs. — The actual position of the heart, and the principal veins and arteries issuing from it, with relation to the lungs, is shown in fig. 312, the names of the several parts being indicated. The lungs are represented as having been pushed to each side, so as to show the heart and vessels more distinctly. z 2 340 ANIMAL PHYSICS. Fig. 312. FRONT VIEW OF THE HEART AND LUNGS. 458. Bronchial Tubes.— Tlie form of the windpipe and its appendages, by which the air is distributed through the lungs, is represented in fig. 313. The vertical part, a, is the windpipe, or trachea, properly so called, and the branches which diverge from it right and left, near the upper part of the lungs, are called bronchial tubes. These diverge lower down and within the lungs into innumerable minute ramifications, as shown at c. One ARTERIES. 341 of the lungs is removed in the figure, to show these ramifications which terminate in the air-cells of the lungs, as the roots of a tree enter the ground. Fig, 313. VIEW or THE WINDPIPE AND BRONCHIAL TUBES, SHOWING THEIR POSITION RELATIVELY TO THE LUNGS. 459. General View of the Arterial System. — From the upper part of the crook of the aorta branches diverge, two of which, bending under the clavicles, descend along the arms, taking the name of the brachial arteries ; and at the point where the aorta descends towards the navel, other branches diverge right and left, descending along the legs, where they take the name of femoral arteries. There are numerous other ramifications, as shown in the general illustration of the arterial system given in fig. 314, where the names of the principal arteries are indicated. 460. Illustrative Diagrams of the Valves. — The mechanism of the valves of the heart, which, as must be evident, play a part of capital importance in the circulating apparatus, will be 342 ANIMAL PHYSICS, more clearly comprehended by the illustrative diagram, fig. 315. Temporal Femoral ( Posterior 1 tibial. Peroneal. Tarsal Carotid Vertebral. Subclavian. Aorta Axillary. — Brachial. Renal Coeliae. Iliac Radial Anterior 1 Tibial ) Fig. 31 J. THEORETICAL DIAGRAM OF THE ARTERIES AN’D THEIR RAMIFICATION’S. VALVES OP THE HEART, 343 Auricle receiving blood from the veins. Vein conducting blood to the ) auricle ) Valve opening from the au- ) ricle to the ventricle. ] Vein conducting blood to the ) auricle. ) Artery from the ventricle. (■Valve opened by pressure from the ventricle, but closed I by contrary pressure. ( Ventricle separated from the l auricle by the valve. Cords to restrain the valve and prevent it from being opened by pressure from the ventricle to the auricle. Fig. 315. THEORETICAL SECTION TO EXPLAIN THE MECHANISM OF THE VALVES OF THE HEART. The form and structure of the valves of the heart will be further illustrated by the diagram in fig. 316. Fig. 316.* SECTION OF THE HEART MADE BY A PLANE PASSING THROUGH THE VALVES, THE AURICLES BEING REMOVED. 1 . Tricuspid valve leading to right ventricle. 2. Fibrous ring surrounding tricuspid valve. 3. Mitral valve leading to left ventricle. 4. Fibrous ring surrounding tho mitral valve. 5. Three valves leading from ventricle to aorta. C. Three valves leading from the right ventricle to the pulmonary artcrv 7. Muscles of the right auriculo-veutricular zono. 8. Muscles of the left auriculo-ventricular zone. 9. Muscles inserted in the aortic /.one. Sappey. 344 ANIMAL PHYSICS. Muscles of the Heart. — When the extensive apparatu- of flexible pipes and tubes, through which the blood must be propelled from the heart to the extremities of the system chiefly by the force imparted to it by the contractile power of the muscles surrounding the auricles and ventricles, is considered, it will be easily conceived that these muscles must be constructed with extraordinary contractile power. Those of the auricles are represented in fig. 317, and those of the ventricles in fig. 318. Fig. 317* MUSCLES OF THE AUEICLES. 1. Bight auricle. I S 8. Muscles surrounding right and 2. Embouchure of inferior hollow left auriculo- ventricular orifices. vein. 9. Muscles surrounding embouchure 3. Embouchure of superior hollow of superior vena cava. vein. 10. Muscles surrounding embouchure 4. Embouchure of coronary vein in of inferior vena cava. right auricle. 11. Muscles separating right from. 5. Left auricle. | left pulmonary veins. ' 6 6. Left pulmonary veins. ' 12 12 12 12. Muscles surrounding em- 7 7. Bight pulmonary veins. i boucliures of these veins. 461. Position of the Heart. — The heart is placed very nearly in the longitudinal axis of the body. Its distance from the point where the neck joins the shoulders, is about one-third of the entire length of the trunk, from which it appeal's, as remarked by Bichat, that the superior members, and especially Sappey. VALVES AND MUSCLES OF THE HEART. 345 the head, are under a much more immediate influence of the heart than the inferior and baser parts. Fig. 318.* MUSCLES OF THE VENTRICLES. 1. Anteiior and superficial muscles directed towards the point of the heart. 2. Muscles of the left ventricle. 3. Anterior and deep seated muscles rising towards the base of the left ven- tricle, after having been twisted round the point of the heart. 4 Twisted muscles directed upwards to the left ventricle. 402. Its Dimensions. — It has been customary to convey a general notion of the magnitude of the heart by comparing it with the fist. This is, however, a loose and erroneous standard, since the magnitude of the hand varies with the exercise and with the manner in which it is employed. The average length of the heart may be stated to be about Sappey. 346 ANIMAL PHYSICS four inches, and its circumference where it is largest is a little Fig. 319.* VIEW OP THE HEART SEEN FROM THE FROXT. Linear dimensions being about seven-eighths of the average magnitude of the heart in an adult. I 7. Trunk of pulmonary artery. S. Aorta. 9. Upper vena cava. 10. Anterior coronary artery. 11. Anterior branch of doronary vein. I 12 12 12 12. The lymphatic vessels. 1. flight ventricle. 2. Left ditto. 3. Right auricle. 4. Appendage of this auricle. 5. Left auricle. 0. Appendage of ditto. less than ten inches. A tolerably correct notion, however, not * Sappey. FORM AND POSITION OF THE HEART. 347 only of its magnitude, but of its form and structure, may be obtained from the eugravings of it given in figs. 319, 320, 321, 322, 323. 7 7 Fig. 320.* VIEW OF THE HEART SEEN FROM BEHIND. 1. Right ventricle. 2. Left ventricle. 3. Right auricle. 4. Inferior vena cava. 5. Superior vena cava. 6. Left auricle. 7 7. Right pulmonary vein. 8 8. Left pulmonary vein. 9. Aorta. 10. Left branch of pulmonary artery. 11. Auriculo- ventricular branch of an- terior coronary artery. 12. Trunk of coronary vein leading to left auricle. 13. Posterior branch of this vein. 14 14 14. Lymphatics. Sappey. 348 ANIMAL PHYSICS. Anatomical Section. — The sections of the heart already given being merely theoretical, and made with the exclusive view of explaining its mechanism, it may be useful here to give the real anatomical section of that important organ. Ftp. 321.* VERTICAL SECTION OP THE HEART, THROUGH THE CAVITY OF THE RIGHT AURICLE. 1. Interior of the right ventricle, showing the fleshy parts which surround it. 2. Part of the tricuspid valve between the right ventricle and auricle. 3. Cavity of tho right auricle. 4. Fleshy parts which surround this auricle. 5. Embouchure of tho great coronary vein, which brings back the black blood from the tissue of the heart. 6. Valve situate at the embouchure of the lower vena cava. 7 and 8. Oval passage, at the bottom of which is placed an opening, which in the ffcetus forms a direct communication between tho two auricles. 9. Embouchures of the upper vena cava. 10. Trunk of the lower vena cava. 11. Aorta. 12. 12 Pulmonary veins. • Sapper. FORM AND MAGNITUDE OF THE HEART. 349 Fig. 322.* VERTICAL SECTION OP HEART MADE TIIROUGII RIGHT VENTRICLE. 1. Ventricular cavity. 2. Tricuspid valve. 3. Tendons inserted in the external surface and edge of this valve. 4. Bunch of tendinous cords, origi- nating directly from the internal side of the right ventricle. 5. Interventricular partition. 6. Mesh-work formed by the inter- section of the fleshy fibres of the right ventricle. 7. Infundibulum. 8. 9, 10. Sigmoid valves of the pul- monary artery. 11. Pulmonary artery. 12. Bight auricle. 463. Structure and Distribution of the Blood-Vessels. — The system of vessels in which the great or systemic circula- tion, as it is sometimes called, is effected, plays a part so impor- tant in the economy compared with that of the lesser or pul- monary circulation, that when the terms arteries, capillaries, and veins are used without any qualifying adjunct, they are * Sappey. 350 ANIMAL PHYSICS, understood to apply exclusively to the vessels of the greater cir- culation, those of the lesser circulation being distinguished as pulmonary arteries, capillaries, and veins. Fig. 323.» VERTICAL SECTION OF THE HEART THROUGH THE LEFT VENTRICLE AND AURICLE. 1. Left ventricle. 2. Mitral valve. 3. Principal fleshy column of the left side, divided into two bundlos, subdivided at their summit. 4. Principal fleshy column of the right side, smaller than the former. 5. Orifice from the ventricle to the aorta. 0. Aorta. 7, S, 9. The three aortic valves. 10. Right ventricle. 11. Interventricular partition. I 12. Pulmonary artery. 13, 14. Its valves. 15, Left auricle. 16, 16. Right pulmonary veins. 17, 17. Their embouchures. IS. Section of the coronary vein going round the left auricle at its posterior part, and arriving at the right auricle. • Sapper. SYSTOLE AND DIASTOLE. 351 464. Capillaries. — Although these vessels have received a distinct name, there is no precise line of demarcation in the economy to mark where the arteries terminate and the capil- laries begin, or where the capillaries terminate and the veins begin. The arteries, as already described, commencing in a single tube, diverge successively in approaching the organs which they nourish into ramifications more and more multiplied in number, and smaller and smaller in their calibre, until at length they assume that extremely multiplied number and minute calibre to which the name capillaries has been given. In like manner, the veins are necessarily as multiplied and minute as are the capillaries, where the capillaries run into them. Nevertheless, although there be no distinct separation between these classes of vessels, it has been found convenient, in descrip- tive anatomy, to designate them by distinct appellations. The capillaries have, moreover, a physiological character distinct from those of either the arteries or veins, being the vessels in passing through which, the bright red or nutritive blood is converted into dark red or vitiated blood, and, therefore, the place where the nutritive functions of the blood are more especially discharged. 465. The Pulmonary or Lesser Circulation. — Analogous ob- servations are applicable to the pulmonary vessels. The pulmo- nary capillaries, by which the blood is diffused through the spongy and cellular substance of the lungs, and by ramifying over the walls of the air-vesicles is exposed to the action of the air by which these are inflated, have no exact point of distinction from the pulmonary arteries on one side, or the pulmonary veins on the other, but are nevertheless distinguished by their physiological functions, since it is in passing through them that the dark blood received from the right ventricle of the heart is converted into the red blood which is propelled through the pulmonary veins to the left amide. 466. Pulsations of the Heart.— The alternate states of the ventricles and auricles during their contraction and relaxation are called systole and diastole, from two Greek words having a like signification. From what has been explained, it will be understood that the systoles of the two auricles are simultaneous and are alternate with those of the ventricles, but the systoles of the auricles do not instantly follow that of the ventricles. Between the systole of the ventricles and that of the auricles the entire 352 ANIMAL PHYSICS. heart is for a moment in a state of repose, or, what is the same, the diastole of both auricles and ventricles has a longer duration than the systole. The mean rate of the pulsations of the heart being about eighty per minute, the duration of one complete pulsation will be three-fourtlis of a second ; and it has been found that the duration of each systole, whether of the auricles or ventricles, is about a quarter of a second, the diastole, there- fore, being half a second. The periodic action of the heart may, therefore, be thus described. The systole of the auricles is performed in the first quarter of a second, during which the ventricles are in diastole. During the next quarter of a second, the ventricles are in systole, and the amides in diastole, and we find during the third quarter of a second both ventricles and auricles are in diastole. A like series of actions then recommences, and is repeated. If we express the state of systole or contractile action by the sign and that of diastole or repose by the sign 0, the simultaneous condition of the amides and ventricles during each complete pulsation of the heart will be represented as follows : Auricles . . . + O O Ventricles . . O _L O It appears from this, that the muscles of the heart are in repose during intervals twice as long as those of their action ; a circumstance which will render somewhat less wonderful the power in virtue of which this organ maintains its action through- out the longest life. The total duration of the action of these muscles will be only one-third of the total duration of life. During the other two-thirds they are in a state of repose. 4G7. The Embouchures of the Pulmonary Veins, which enter the amides, are not, like those of the arteries proceeding from the ventricles, supplied with valves, by which the reflux of the blood during the auricular systole would be prevented. That reflux, however, is resisted partly by the contraction of the amide itself, which, commencing near the embouchures of the veins, presses before it the blood, partially closing, these embouchures, and pushing the blood before it nearlj in the BEATING OP THE HEART. 353 same manner as the food in process of digestion is pushed through the intestine. The valves already described between the auricle and the ventricle being large and freely opened, the blood passes directly into the ventricle, exerting no force of reflux upon the embouchures of the pulmonary veins. The veins leading into the right amide, are supplied with valves called the Eustachian and coronary valves, which, how- ever, close them but imperfectly ; for the reasons just men- tioned, however, there is no reflux. 468. The Contractile Force of the Cardiac Muscles must necessarily vary, like all forces in well regulated mechanism, in proportion to the mechanical effect which it is required to produce. Either a deficiency or redundancy of impulsive force would soon produce a derangement of the organ. But from the position of the heart with relation to the lungs on the one hand, and to all the organs of the body to which it propels the nutritive blood on the other, it will be apparent that consi- derably more force is required to maintain the greater than the lesser circulation ; and, consequently, the power of the muscles which act upon the left auricle and ventricle must be propor- tionately greater than those which act upon the right. Various means have accordingly been suggested by which the compa- rative power of these muscles can be estimated ; the most simple and practicable of which is the comparison of the weight of the two sets of muscles. By such a comparison, it has been ascertained that the weight of the muscles which act upon the left side of the heart is twice that of the muscles which act upon the right side. This proportion is found to be sensibly the same in man, the horse, the sheep, the dog, the cat, the rabbit and the pig. It is further remarkable, that this predo- minance of weight is found to prevail especially in the ventricles, a result which is consistent with the fact that the ventricles are the more immediate propelling agents. The left ventricle, therefore, having twice the weight of the right ventricle, has consequently twice the propelling power ; and the greater circu- lation, accordingly, is maintained by a moving power greater than that which maintains the lesser circulation in the pro- portion of their respective resistances. 469. The Beating of the Heart is a phenomenon which has not been fully and satisfactorily explained by physiologists. 354 ANIMAL PHYSICS. It is certain, however, that whatever be its cause, the organ, being fixed at its upper part in the region of the auricles, is free in the lower part, which is the region of the ventricles ; and that the effect familiar to every one as the beating of the heart is produced by alternate motion forwards and 1 jack- wards of the lower part of the organ, which takes place simul- taneously with the ventricular systole and diastole. 470. Torsion of the Heart.— But, besides this forward motion, it has been ascertained that the heart has also an alternate motion of torsion round its vertical axis ; so that, if it were denuded and exposed to view in the living subject, it would present itself from moment to moment under different aspects to the eye, just as a body would which turns round vertically, with an alternate motion right and left. At the moment of the ventricular systole the heart turns slightly on its axis from left to right ; and during the diastole, turning in the contrary direction, resumes its first position. This move- ment of torsion is not extended to all parts of the heart, the auricular region having no share in it. The torsion commences in the horizontal plane, passing through the valves, where it is scarcely perceptible, and increases gradually from that to the point where it is greatest. This movement of torsion involves also a slight forward pro- jection of the point of the heart, which must not, however, be confounded -with the much more considerable movement of the organ which produces the beating. 471. The Force with which the Blood is propelled through the arteries has been measured by direct experiment, by putting it in communication with a mercurial gauge inserted in an opening made in the aorta for the purpose. In this manner, it was shown to be in equilibrium with a column of six inches of mercury. Blood being about fifteen times fighter than mercury, it follows that if a siphon-gauge be inserted into an artery, a .column of blood would rise in the vertical length to the height of about seven and a half feet, at which it would be sustained by the tension of the fluid contained in the arteries. 472. The Structure of the Arteries. — The action of the muscles and valves of the heart would lead to the supposition CIRCULATION NOT INTERMITTING. 355 that the movement of the blood in circulation must be inter- mitting, and not continuous. It might be inferred, that during each systole, the blood would be pushed forward through the arteries, capillaries and veins ; and that, during each diastole the motion would be suspended. This is, however, not the case in the animal economy, the motion of the blood being continuous and not intermitting. Though continuous, it is not strictly uniform, being accele- rated and retarded by the alternate action of the ventricular muscles. This continuous movement of the blood is produced by a provision in the structure of the blood-vessels. The arteries are flexible tubes composed of three coatings, the innermost or first of which is a thin and extremely smooth membrane which lines the ventricle, and is adapted to allow free flow to the current of the blood. This tube is sheathed in another, consisting of a thick, yellowish, highly elastic sub- stance, of annular structure, and of involuntary muscular fibres, the rings composing it having their planes at right angles to the direction of the artery. This is again invested with an external coating of dense and close cellular texture. Thus, the structure of the arteries may be said to resemble that of the hose of a fire-engine. The arteries thus constructed are endowed at once with elasticity and muscular contractility, both of which qualities have an important share in maintaining the circulation. During the ventricular systole, the blood is propelled with great force into the arteries, which, in virtue of their elasticity, yield to this momentary pressure, and are augmented in their calibre. During the ensuing diastole, when the valve between the ventricle and aorta called the sigmoid valve is shut, the blood in the arteries, no longer receiving any impul- sive force from the heart, would cease to move, so that the cir- culation, instead of being continuous, would be intermittent. This effect, however, is prevented by the elasticity of the coats of the arteries, which re-act upon the blood that has distended them, so as to urge it in one direction or the other ; but since its reflux to the heart is prevented by the sigmoid valve, which opens only to the arteries, the blood must be pressed by the elasticity of the arterial coats towards the organs which it is intended to nourish. The elasticity of the arteries therefore acts in this case pre- cisely in the same manner as does a fly-wheel attached to any machine propelled by an intermitting power. The motive power a a 2 356 ANIMAL PHYSICS. exerted by the ventricle in its systole is expended partly in pro- pelling the blood into and through the arteries, and partly in dis- tending the elastic coats of the arteries. During the succeeding diastole, the latter part of the moving force is given back to the blood by the compression produced in virtue of the elas- ticity of the arteries, and the blood is accordingly urged forward through the arterial system during the suspension of the moving power of the heart, just as a machine continues to be driven by the inertia of the fly-wheel during the inter- mission of the power which drives it. If it might be supposed that the force of the heart during the systole is equally shared between the blood and the coats of the heart which it distends, then the movement of the blood would be perfectly uniform, notwithstanding the intermitting action of the ventricle. That this equality of action, however, does not obtain, is proved by experiments in which the force of the blood is directly measured ; from which it appears, that during each ventricular systole, the column of mercury sup- ported by the blood is raised about a 12th part of its entire height. But, besides the elasticity of the arteries, which may be regarded as a passive force called into action by and depending on that of the ventricular systole, the arteries have been stated by many physiologists to possess muscular contractility, in virtue of which the blood is constantly propelled forwards quite independently of any action of the heart. It will therefore be apparent that the circulating mechanism is not to be regarded as being analogous to an hydraulic system with a moving power at one end only, but rather to an hydraulic apparatus, in which, besides the primitive moving power by which the liquid is pro- pelled into the tubes, the tubes themselves throughout their entire length exercise a mechanical pressure on the included liquid, to which effect is given by the reaction of the sigmoid valve preventing the reflux of the blood, which is consequently driven forward towards the capillaries. One of the effects which render manifest this contractile force of the arteries is the fact, that after death, when the action of the heart ceases, all the blood contained in the arteries is driven forward into the veins. 473. The Pulse is nothing more than the alternate disten- sion and contraction of an artery which takes place during the systole and diastole of the ventricle. VELOCITY OF THE BLOOD. 357 474. The Capillaries. — These vessels, as already explained, are the minute canals which traverse the organs and are inter- posed between the arteries and the veins, receiving the blood from the former and discharging it into the latter. They are the immediate theatre of nutrition, since it is in them and through their coats that the blood imparts nourishment to the organs, and becomes impregnated on the other hand with matter which the organs reject. In this process the blood imdergoes an important change in its physical properties, being converted in its colour from a bright vermilion red to a dark brown red, and losing altogether its nutritive functions. Having undergone this change, it is discharged by the capil- laries into the veins, and through them flows back to the right side of the heart. The capillaries, like the arteries, are endowed at once with elasticity and contractility, and possess the latter property even in a greater degree than the arteries. If the blood-corpuscles were perfectly solid and inelastic, it is evident that, as they must pass through the capillaries, their magnitude would impose a minor limit upon the calibre of these vessels, since a corpuscle cannot pass through a tube whose calibre is less than its own diameter. But, since the corpuscles are more or less elastic or compressible, they sometimes by elongating themselves force their way through tubes whose calibre is a little less than their own diameter. The most minute capillaries have a calibre measuring about the 4000th of an inch, while the calibre of the largest of these vessels measures the 2500th part of an inch. The capillaries which pervade any given organ are of uniform calibre, but those w'hich pervade some organs are of greater calibre than those which pervade others. The largest capillaries are those which run through the bones and mucous membranes, and the most minute those which pervade the nervous system, the lungs, the skin, and the muscles. The rate at which the blood moves through the vessels must decrease in proportion as the collective magnitude of the transverse sections of the vessels increases, just as the current of a river is diminished in speed as its bed is enlarged, or as the collective section of the branches which form its delta is aug- mented. Now, it is found that the collective section of the ramification of the arteries is greater than that of their trunks, and the collective section of the capillaries is greater than that of the arteries at the point where the branches of the latter 358 ANIMAL PHYSICS. enter them. The current of the blood, therefore, most rapid in issuing from the heart, continually decreases in speed as it approaches the capillaries, through which it passes at a still slower rate, in proportion to the increased magnitude of their collective transverse sections. This efiect is quite in accordance with the physiological functions of the capillaries, the retardation of the blood giving the time necessary for the production of those physical changes which necessarily accompany the discharge of its nutritive functions. Since the vascular system is always completely filled with liquid blood, and since the blood, like all other liquids, is inelastic and incompressible, it follows obviously that the quantity of blood propelled into the arteries by each ventricular systole is exactly equal to that which flows during each pulsa- tion of the heart from the arteries into the capillaries. But, since the blood thus propelled from the heart is distributed among the capillaries of various parts of the body, and since the capillary vessels are subject, from various causes, to occasional obstruction, a redundancy of blood will sometimes be dis- charged into certain capillaries, others being at the same time deprived of a corresponding quantity. Among the causes which produce such local variations of the circulation, are the nervous actions which attend mental emotions. Thus, shame or anger, by compressing certain capillaries, propel the blood in undue quantity through those of the cheeks, producing the blush of shame, and the suffused red of anger. On the other hand, the emotions of terror and despair produce obstructions which are attended with the pallor that indicates these emotions. 475. The Veins, like the arteries, are flexible tubes, similar in then- internal and external coating ; but the intermediate envelope of annular structure is replaced by a thin coating of longitudinal, loose, and extensible fibres. One of the consequences of this difference of structure is, that an artery, even though empty, as it is after death, pre- serves its tubular form, while an empty vein will collapse. The veins are elastic and contractile, but much less so than the arteries. If a vein be temporarily distended, it will, when relieved, recover its former calibre, but if the distension be long continued, the enlargement will be permanent. This permanent enlargement of the veins is very common with aged persons. The pressure of the blood in the veins is much less than in ARTERIES AND VEINS. 359 the arteries. Being measured by means similar to those already described, it is found to be balanced by a column of mercury measuring six-tenths of an inch, while the pressure of blood in the arteries balances a column of six inches. The tension, however, of blood in the veins is subject to much variation. Thus, M. Mogk found that in the jugular vein it balanced half-an-inch of mercury ; in the brachial vein, six-tenths of an inch ; and in the crural, nearly an inch. 47 6. The Valves of the Veins. — Considering the great length Valve Valve Valve. Fit'. 324. A VEIN', SHOWING THE VALVES WHICH RESIST THE REFLUX OF THE BLOOD TO THE CAPILLARIES. and numerous and complicated ramifications of the arteries, it ABC Fig. 325. will be evident that the single point of reaction opposed to the 360 ANIMAL PHYSICS. power propelling the body by the sigmoid valve, would liave but imperfect efficiency ; and this efficiency would be still less when the propulsion of the blood back to the veins is considered. This imperfection is accordingly foreseen, and removed by the provision of a series of valves called, from their form, semi- lunar valves, in the veins, all of which, opening towards the heart, prevent the reflux of the blood towards the capillaries. A vein cut open so as to show the form of these valves, is represented in fig. 324. These valves are formed by folds of the lining membrane of the vein, and are strengthened by some fibro-cellular tissue included in it. They project obliquely into the vein, and two folds or flaps are generally placed opposite each other, as shown at a, fig. 325, which represents a part of a vein laid open and spread out with two pair of valves. The longitudinal section of a vein, showing the opposition of the edges of the valves in then- closed state, is represented at b ; and a portion of a distended vein, exhibiting a swelling in the situation of a pair of valves, is shown at c. 477. Cicatrisation of Wounds. — From the facility of healing a wounded vein, and the much greater difficulty of performing the same office for a wounded artery, it is obviouslj* desirable that the latter class of vessels should be more espe- cially protected from injury by external causes. Nature has accordingly placed them in such situations in the economy of the system as to afford them in all cases the greatest amount of protection. While a large portion of the veins are super- ficial, the arteries, on the contrary, are deeply lodged within the surface. In all cases where the blood-vessels must be carried over joints — such, for example, as the knees, elbows, and shoulders — the arteries are conducted within the bend of the articulation ; those of the anus passing within the arm-pit ; those of the elbow, within the inflection of that joint, and those of the leg within the inflection of the knee ; while the vessels which pass over the shoulder, the external angle of the elbow, and the knee-cap, are generally veins. The current of the blood, considered in relation to the ramifications of the blood-vessels, is contrary in the veins to that in the arteries. In the former, it flows from the branches to the trunk, and in the latter from the trunk to the branches. The circulation taken collectively has been com- pared to a tree, the extreme ramifications of whose branches MICROSCOPIC VIEW. 361 should be brought iuto connection with the extreme ramifications of its roots. 47 8. The Circulation of the Blood is rendered visible in the Microscope by extending any thin, semi-transparent mem- brane of a living animal before the object-glass of the instrument, and throwing a sufficiently strong light be- hind it. The arterial and venous currents of the blood will then be distinctly seen by the aid of a moderate magnifying power. The best method of making this experiment is by means of the web part of the foot of a frog, or of the thinnest part of its tongue. In figure 326 the circulation in the blood- vessels of the foot of the animal is thus shown, where a a are arteries in which the blood is seen flowing, as indicated by the arrows, from the trunks to the branches, and v v, veins, in which it flows from the branches to the trunk, the capillaries being represented by dotted lines. 479. Daguerreotypes of the Circulation in the tongue of a frog were produced by Messrs. Donne and Foucault, in the same manner as already described in the case of the corpuscles of the blood. We have, by the permission of these gentlemen, reproduced, in fig. 327, one of these. When the tongue is viewed, placing a light behind it, with a simple mag- nifying glass having a power no greater than fifteen or twenty, the observer will be filled with astonishment at the magnificence of the spectacle. To imagine a geographical map to become suddenly animated, by their proper motions being imparted to all the rivers delineated upon it, with their tri- butaries and affluents, from their fountains to their embouchures, would afford a most imperfect idea of this object, in which are rendered plainly visible, not only the motion of the blood through the great arterial trunks, and thence through all their branches and ramifications to the capillaries, but also its complicated vorticular motions in the glands, its return through the smaller ramifications of the veins to the larger trunk veins, and its departure thence en route for the heart. Such is the astonishing spectacle, circumscribed within a circle having the diameter of the 120th of an inch, magnified, however, 400 times in its linear, and therefore 160000 times in its superficial dimensions. The arteries are distinguishable from the veins very readily, by observ- ing the direction in which the blood flows, its velocity, and their comparative calibre. In the arteries the blood flows from the trunk to the branches ; 362 ANIMAL PHYSICS. the arrows show its course passing into the principal branches, 1, 2, and 3, from which it flows into all the smaller arterial ramifications. The course of the blood in the veins, on the contrary, is from the branches to the trunk. Fig. 327. MICROSCOPIC DAGUERREOTYPE OF PART OF THE TONGUE OF A FROG, THE 120TH OF AS INCH IN DIAMETER. The greater arteries are accompanied by a greyish flexible cord, which can be perceived, but not without some attention : it passes along the sides of the artery : this cord is only a nerve. As the ramifications of the arteries are multiplied in number they are diminished in calibre, and merge at length in the capillaries, from v liieh they are scarcely distinguishable, the latter being equally indistinguish- able from the smaller veins. As the capillary vessels diminish in diameter, the red corpuscles at length so completely fill them, that they can only move in them one by one, and they can be thus seen following one another at perceptible intervals. If the microscope be directed to that part of the edge of the tongue, which is within the limits of the hole made in the cork on which the tongue is extended, the blood can be traced in its THE SKIN. 363 course to the extreme arteries, and thence from the smaller to the larger veins on its return to the heart. The vascular system of the tongue appears traced upon a greyish semi- transparent ground, on which a multitude of fine fibres, v v, are seen ex- tended in different directions ; these, existing at different depths within the thickness of the tongue, appear superposed and interlaced ; these fibres belong to the muscle of the organ, and their characteristic action is rendered evident under the microscope by their alternate contraction and extension. A number of greyish spots, somewhat round in their outline and a little more opaque than the neighbouring parts, appear scattered through the tongue : these spots belong to the mucous membrane, and are in fact the mucous follicles of the tongue, the little glands in which is secreted the viscous humour which coats in such abundance the tongue of the frog ; and we accordingly find that if it be wiped off, it will be almost immediately reproduced. They are the theatres of a surprisingly compli- cated and active blood-motion. The sanguineous fluid enters them at one side, generally by a single small artery, rarely by two ; and following the course of this artery, it pursues a nodulated path, resembling the form of a bow of ribbon, or the figure 8, and issues from them at a point opposite to that it entered. The organs of which we speak, says Dr. Donne, having a certain thickness, we cannot always see at once the entrance and departure of the blood, the point of its departure being often in a plane inferior or superior to that of its entrance, and the two points not being, therefore, at the same time in focus. But in any case, nothing can be more curious or more profoundly interesting than the vortices of rapid circulation, thus exhibited, in a space so circumscribed and within the limits of an organ, which is evidently one of secretion. 480. General Ramifications of the Blood-vessels. — As lias been already observed, tbe arteries and veins distribute them- selves in innumerable ramifications from tbe heart tbrougb all parts of tbe body. They spread tbrougb tbe muscles, amid whose fibres they ramify, and penetrate the very bones, whose structure is filled with them. We have already given, in fig. 314, a general view of the manner in which the arteries ramify ; and the veins differ from the arrangement there represented only in this, that while the larger arteries are generally confined to certain depths within the surface, veins of large size ramify superficially also in immense numbers, so as to be rendered visible in various parts of the body as blue lines seen through the semi-transparent texture of the epidermis or superficial skin. 481. It may be here stated that the skin, properly so called, is that covering having a very sensible thickness, which is taken between the fingers when we pinch the sur- face of the body, as, for example, the back of the hand or the neck. This consists of two distinct parts ; the inner and thicker part, called the true skin, cutis , or derma, a Greek :364 ANIMAL PHYSICS. word signifying the skin, and the superficial or thinner part called the cuticle, epidermis, or covering of the skin, from a Greek word in\ ( epi ), signifying upon or over. The membrane thus named is that which is raised and separated from the derma by a blister. Although it would be impossible to give, in a work like the present, figures which would convey any notion of the local distribution of the blood-vessels through the body, other than the general representation of the arterial system given in fig. Fig. 32S. 314, it may, nevertheless be interesting to readers who are nni professionally medical, to see the wonderful structure of the BLOOD-VESSELS OF THE HEAD. 365 vascular system in some of the principal parts of the human economy. 482. Blood-vessels of the Mesentery. — In fig. 328 are represented the trunk (1), and the innumerable ramifications and anastomoses (2, 3, 4, 5) of the artery which spreads over the mesentery, a membranous structure connected with the intestines. 483. Blood-vessels of the Head and Face. — In figure 329 are shown some of the principal arteries which overspread the head and face, to each of which anatomists have assigned proper names. Fig. 329.* The external carotid trunk, which passes up on the inner Sappey. 366 ANIMAL PHYSICS. side of the jaw, is shown at 1 ; the branches which overspread the head at 2 ; the temporal ramifications at 3 and 4 ; the arterie- Fig. 330.* Fig. 331.* which go to the teeth at 6 ; those which go to the mouth at 7 ; those of the palate at 8 ; those of the eye at 9, and so on. • SapPey. BLOOD-VESSELS OF ARMS AND LEGS. 367 484. Blood-vessels of the Arm and Hand. — The principal arteries of the arm and hand are shown in fig. 330, and the superficial veins in fig. 331. In both figures the veins and arteries are indicated by numbers, and the muscles by letters. In fig. 330, the artery, 1, called the humeral or brachial artery, passes down the arm, and within the hollow of the elbow is pro- • Sappey. 36 8 ANIMAL PHYSICS. tectecl by the tendon of the biceps muscle, a. Immediately below the elbow-joint it diverges into two, one of which, 4, 4, passes over the radius, and the other, 6, under the muscles, f, a, u ; each then descends to the palm of the hand, and there forms a superficial and a deep arch, which sends branches, 8, 8, 8, 8, down each of the fingers. In fig. 331, the superficial veins which carry back the blood from the hand and arm, run into each other like tributaries feeding a large river, the veins increasing in calibre until they terminate in the great vein, 9, which passes under the shoulder. 485. Blood-vessels of the Beg and Foot.— In like manner in fig. 332 are represented the principal arteries, and in fig. 333 the principal veins which ramify over the leg and foot. 486. The Circulation in the Foetus must differ essentially from that which takes place after birth, the process of respira- tion by means of the lungs being as yet dormant. Instead of passing wholly through the right ventricle and the vessels of the lungs and thence to the left auricle, the blood in the right auricle passes in part to the left auricle directly through a temporary opening in the septum which separates them, and in part into the right ventricle, and thence into the pulmonary artery, from which the greater part escapes through a conduit, called the ductus arteriosus, into the descending part of the aorta. A small part of the blood, however, passes from the right t entricle to the lungs. The blood propelled from the left ventricle circu- lates generally throitgh the system upwards as well as downwards, but that propelled from the right ventricle through the ductus arteriosus passes exclusively to the lower parts of the body. Immediately after birth, respiration commencing, the blood finds its way into the right ventricle and through the pulmonary vessels ; the lesser circulation commences, and is continued during life ; and the temporary opening in the septum being no longer required, is gradually closed. In rare cases, however, it remains open during life, in which case the blood in the systemic circu- lation is more or less impure. 487. The Circulation in Mainmifcrs generally is in all essential particulars similar to that which obtains in the human economy. In all these the heart is composed of two compart- ments perfectly distinct, each of which has its own auricle and ventricle ; and each of them, as in man, lias the double circula- tion, the great and the lesser, the systemic and the pulmonary. CIRCULATION IN BIRDS. 369 488. The Blood of Birds is more rich in reel corpuscles than that of mammifers, and, instead of being circular, they are ellip- tical : their diameter is also greater. The circulation in birds differs in nothing essential from that of mammifers. The blood passes from the left ventricle of the heart into the arteries, by which it is distributed to all the organs ; and after passing through the capillaries, it returns by the veins to the right auricle, from which it passes to the right ventricle. It is thence propelled to the lungs through the pulmonary arteries, and returns by the pulmonary veins to the left auricle. The heart is similar in form, structure, and position, to that of mammifers. The aorta, after leaving the heart, is divided into three large trunks, two of which carry the blood to the head, wings, and chest ; and the third, being bent downwards round the right bronchial tube, is analogous to the descending aorta. The venous system is terminated by three large trunks, one of which is the analogue of the venae cavae of mammifers, and the two others correspond nearly to the two innominate or brachio- cephalic veins, which however do not combine so as to form a single superior vena cava, as in mammifers. Pulmonary circulation. Fig. 334. THEORY OF CIRCULATION' IN MAMMIFERS AND BIRD3. The general circulating system of birds will be better under. n n 370 ANIMAL PHYSICS. A. B. C. D. E. P. G. H. I. K. L. M. Fig. 335. ARTERIAL SYSTEM IN BIRDS. The aorta. The carotid artery. A branch of the aorta which, after having supplied the carotid and subclavian arteries, throws out ramifications over the muscles of the chest, and is analogous to the mammary artery in mammifers. One of the branches of the vertebral artery going to the muscles of the shoulder. Ramifications of the external carotid artery. The lingual artery. The artery of the trachea. The renal arteries. The femoral arteries. The iscliiatic artery, going to the inferior members. The sacral artery, being a continuation of the aorta, and rannn ing to form the inferior mesenteric artery and other small vessels. The cloaca. CIRCULATION IN REPTILES. 371 stood, after wliat has been explained, by the theoretical diagram, tig. 335. The circulation in mammifers and birds will be rendered more intelligible by the theoretical diagram, fig. 334, where the vessels containing the venous blood are darkly shaded ; those containing the arterial blood being white, and the capil- laries being indicated by the dotted lines between the termina- tions of the arteries and veins. 489. The Circulation of Reptiles is incomplete, or rather, has a mixed character, being intermediate between the single circulation of mammalia before birth and the double circulation after it. The heart of reptiles has but one ventricle, which' communicates at once with the pulmonary artery, the pul- monary vein, and the aorta or primary artery of the great circulation. A part only of the venous blood entering from the right auricle, passes into the pulmonary artery, and circulating through the lungs, returns through the pulmonary vein. This portion only, therefore, is reconstituted, and returning through the pulmonary vein, is again discharged into the single ven- tricle, whence, mixed with the other part of the venous blood Pulmonary circulation. Fig. 33C. THEORY OF THE CIRCULATION IN REPTILES. which does not circulate through the pulmonary vessels, it passes into the aorta and through the vessels of the great u b 2 372 ANIMAL PHYSICS. circulation. These observations will be more clearly appre- hended by reference to the diagram, fig. 330. 490. Circulation in the Lizard. — The entire circulatory CIRCULATION IN REPTILES. 373 apparatus, as it exists in reptiles generally, is shown in fig. 337, which represents that of a lizard. It will be observed that the aorta, which in the human economy and in that of other mam- malia, after ascending from the heart, is bent downwards, descending on the left side, is in this case double, being bent both to the right and to the left ; the two branches reuniting, however, below the heart, and going vertically downwards. The arrangement of the circulatory apparatus varies in different reptiles, but there is always a direct communication between the vascular apparatus of the arterial and that of the venous blood, so that these liquids are mixed in the ventricle of the heart which supplies the arteries. The consequence is, that the blood which circulates is imperfectly arterialised. The heart is almost invariably constructed with two auricles and a single ventricle, as shown in fig. 338. The arterial blood coming from the lungs is received in the left, and the venous blood coming from the capillaries in the right auricle. Both these auricles discharge their contents into the common ven- tricle, from which one part of the mixed blood is sent through the system by the aorta, and the other through the lungs by the pulmonary arteries. 491. The Circulatory Apparatus of the Tortoise, represented in fig. 338, presents an example of this, and after the explanation just given will be easily understood. Right branch of the aorta Ventral aorta. Bight pulmonary artery Right pulmonary vein Right auricle Vena Left branch of the aorta Left pulmonary artery. Left pulmonary vein. Left auricle. Single ventricle. Fig. 338. CIRCULATORY APPARATUS OF THE TORTOISE. In certain exceptional cases, however, reptiles are furnished with a more perfect circulatory apparatus, the single ventricle being divided into two compartments by a membranous partition, which prevents the intermixture of the arterial and venous blood within the heart. Nevertheless, even in these cases, a mixture does take place, but at some distance from the heart 374 ANIMAL PHYSIC.4-, in consequence of which one half of the body receives blood which is [only imperfectly arterialised. In fact, the venous blood poured into the right com - partment of the ventricle is not propelled exclusively to the lungs as in warm- blooded animals ; for near the opening of the pulmonary arteries another vessel is found, which also proceeds from the right ventricle, and which, after being bent behind the heart, enters the descending branch of the aorta, and consequently mixes a portion of the venous with the arterial blood. 492. The Crocodile, whose circulatory apparatus is shown in fig. 339. presents an example of this. V F G G C V Fig. 339. CIRCULATORY APPARATUS OP THE CROCODILE. Veins which bring the blood from all parts of the body to the right auricle E'. The ventricle divided into two compartments, right and left, by a partition. The two pulmonary arteries, which go from the ventricle to the lungs. The vessel which goes from the right compartment of the ventricle into the descending branch of aorta. The pulmonary veins which bring the arterial blood from the lungs to the left auricle. The left auricle from which the blood passes into the left compart- ment of the ventricle. The right auricle. The aorta into which the blood passes from the left ventricle. The two arteries which supply the head and the anterior part of the trunk, which receive pure arterial blood, since the vessel F only enters the aorta at a point below Chose from which the vessels G G issue. 493. Circulation in Fishes. — The circulatory apparatus of fishes is still more simple, the heart having but one ventricle and one auricle. The blood is propelled from the ventricle through the gills, or respiratory apparatus (which will be more fully explained hereafter-), where it acquires the nutritive V V. A. B. B'. C. D. D'. E. E'. F. GG. CIRCULATION IN FISHES. 375 property, and from thence it passes directly, without returning to the heart, through the vessels of the great circulation ; and, after passing through the capillaries, it returns to the heart, enters the auricle, and passes from thence to the ventricle, and so on. This apparatus is represented by the theoretical dia- gram, fig. 340. Pulm. circulation. Systemic circulation. Fig. 346. Veins Ventr. Aur. Heart. Dorsal art. THEORY OF CIRCULATION IN FISHES. Although the structure of the heart in fishes is so much more simple than in mammalia, the double circulation is nevertheless equally complete, and even more so, than in reptiles. In the latter class, a part only of the black blood circulates through the lungs, while in fishes the whole of it passes through the equivalent organ. The difference between fishes and mammalia in respect of circulation consists merely in this, that the pro- pelling machine has less power ; while in mammalia and birds there are two propelling machines — one for propelling the blood through the systemic vessels, and the other for propelling it through the pulmonary vessels — in fishes, the same machine propels it through both. This difference may perhaps be explained by the fact that the less complex form of fishes requires a proportionally less powerful impulse to be given to the blood to enable it to return to the heart. The arrangement of the circulatory apparatus will be understood by reference to fig. 341, where it will be seen that the heart is placed under the throat in a cavity separated from the abdomen by a sort of diaphragm, and surrounded and protected by the bones of the pharynx, the branchial, and the shoulder. It is composed of a single auricle, which receives the venous blood collected in a large receptacle near it, and of a single ventricle below, from which a branchial artery issues, the base of which 376 ANIMAL PHYSICS. being enlarged, forms a contractile bulb. This vessel immediately diverges into two lateral blanches, the ramifications of which are distributed through the branchiae or gills, which, as will hereafter appear, are invested with the functions of the lungs. After traversing these organs, the blood remounts towards the head by another vessel, which is carried along the border of the branchial bones. After sending some branches to the neigh- bouring parts, these vessels reunite to form a great dorsal artery, which is directed backwards under the vertebral column, and throws out branches CIRCULATION IN THE LOWER CLASSES. 377 to all parts of the body, as is shown in the figure. In returning to the heart, all the venous blood does not proceed directly to the receptacle leading to the auricle just mentioned ; that which circulates in the intestines and some other parts being conducted into the liver before returning to the heart. It appears, therefore, that as in mammifers and birds, the whole of the blood traverses the branchial organ, which is the equivalent of the lungs, but passes only once through the heart, in consequence of which its rate of circulation is slower. It is evident, from what has been stated, that the functions of the heart itself correspond to those of the right compartments of the same organ in the superior species of vertebrated animals. 494. The Circulation of Insects is very imperfect. The blood is not included in a special vascular system. There are neither arteries nor veins, and the nourishing fluid diffuses itself in the interstices of the organs and tissues. Nevertheless, it is animated with a certain movement having the character of circulation, produced by a dorsal vessel placed upon the median line of the body, immediately above the digestive tube. The blood is aqueous and colourless. It enters the pro- pelling apparatus by lateral openings furnished with valves to prevent its reflux, and issues from it at the cephalic extremity. Its motion, however, does not solely depend upon this organ, since in several insects a sort of moveable valve has been dis- covered, the action of which imparts a rapid motion to the cur- rent of the blood : it is remarkable that it is in the legs that this apparatus is fixed. 495. The Circulation of Arachnida is less imperfect than that of insects. As with all animals of this class the blood is white, but some arachnida are provided with a circulatory appa- ratus. The heart placed upon the back, having an elongated shape, sends out various arteries, and the blood, after having traversed the organs, goes to the lungs, from which it returns to the heart. With other arachnida, however, the circulation is much more imperfect, the apparatus being in all respects similar to that of insects. 496. The Circulatory Apparatus of Crustacea consists of a heart with one ventricle, without an auricle, by which the blood is propelled through tho arteries, capillaries, veins, and lungs, or substitutes for them. The veins, however, consist of irregular cavities, which do not take the usual vascular form, 378 ANIMAL THYSICS. and which constitute around the branchial canals a kind of venous reservoir. The venous blood, after thus bathing these organs, and recovering its nutritive properties, passes into tubes, which, corresponding with the pulmonary veins of superior animals, cany it to the heart. The arterial circulation is there- fore systemic, and the venous respiratory. In the case of Crustacea, such as lobsters, crabs, crayfish, and other animals of the same, class, the theory of the circulation, is represented in fig. 342. Pulm. circulation. Heart. Arteries and capillaries. Systemic circulation. Fig. 342. THEORY OP CIRCULATION IN CRUSTACEA. 497. The Circulation in IVIollusca differs from that of fishes in nothing but the position assigned to the heart, which is placed in the current of black instead of red blood. In fact, if the heart be imagined in fig. 340 to be transferred from the middle of the veins where it is on the left of the figure to the middle of the dorsal artery on the right, the figure will then represent the theory of the circulation in mollusca. It will be observed, that in that case, the blood which passes through the heart will be red instead of being black as in fishes. The actual circulatory apparatus will be more clearly com- prehended by reference to fig. 343, iu which the general CIRCULATION IN THE LOWER CLASSES. 379 anatomy of a snail is represented, the parts being as follows : — The heart of this class of animals is usually composed of a single ventricle h, from which the arteries i issue ; the auricles are sometimes double and sometimes single, and receive the red blood from the vessels o o, which may be regarded as pulmonary veins coming from the respiratory apparatus d d, to which the blood is conducted directly by canals more or less complete, having the venous character, such as n n. This is the case with snails, oysters, and all other mollusca of the gasteropodous and acephalous classes. There are, however, exceptional cases, in which no auricles are found. Fig. 343. THE CIRCULATORY APPARATUS OP THE SNAIL. a. The mouth. b. The foot. c. The auus. d d. The lungs. e. The stomach covered salivary glands. //. The intestine. <7. The liver. h. The heart. i. The aorta. j. Gastric artery. 1. Hepatic artery. 498. The Circulation of Zoophytes is still more imperfect. In some of these, the holothuria and sea urchin, for example a series of canals is discoverable, in which the blood circulates ; in others, such as the Medusa, the circulation is performed by appendages of the digestive tube. There are others in which the blood or nutritive fluid is diffused into the tissues by a sort °f infiltration through the coats of the digestive tube, without any other discoverable mode of distribution. k. Artery of the foot, ni to. The abdomen, having the function of a venous sinus. by the nn. An irregular canal commu- nicating with the abdo- men and sending blood to the lungs. o o. Vessels which carry arterial blood from the lungs to the heart. 380 ANIMAL PHYSICS. CHAPTER VII. THE LYMPHATICS. 499. Absorption is the name given to the process by which the organism receives from external sources the matter necessary to repair its waste and supply its growth. All matter taken into the system by this means must be in the fluid state, and food which is not naturally in that state must be reduced to it previously to its absorption by solution in the juices of the organs. 500. Absorption may be exhibited experimentaUy by various simple expedients. If a frog be immersed in water so as to prevent the water from entering its body except by the skin, it will be found after a certain 'time that a considerable quantity of water will have been imbibed by it, as will be proved by its increased weight. In like manner, a known volume of water being introduced into the stomach of a dog, all the passages from the stomach to other parts being previously closed by ligatures, the water will, after a certain interval, disappear, having passed through the membrane of the stomach to other parts of the system. This process of absorption is ascribed to the physical agency already referred to (447), called endosmose and exos- mose, and not to any such mechanical process as that by which water passes through the meshes of a sieve, or into the cellular interstices of a sponge. 501. Lymphatics. — Animals of the lower orders which have imperfect circulating apparatus derive their entire nourishment from this sort of cutaneous absorption ; but in classes supplied with a less imperfect circulatory mechanism, the function of nutrition consists, first, of absorption through the coats of the blood-vessels, and secondly, of the transfer of the nutritive matter by the current of the blood to the various parts of the organism. In the highest forms of organisation the process is still more complex. There the chief part of the absorption is performed by a vascular system specially appropriated to LYMPHATICS. 381 that function, called the lymphatics, already mentioned (453). 502. Thoracic Ihict.— The lymphatic vessels ai’e systems of membranous tubes resembling the veins, but generally smaller, which pervade almost every part of the organism, following for the most part the same route. They generally run into each other like tributaries into a river, and then combining, form tubes of larger and larger calibre, until at length they form trunks of considerable diameter. The principal part of these vessels, converging from all parts of the body, finally coalesce, and discharge their contents into a large trunk, called the thoracic duct or canal, situated in front of and parallel to the spinal column. This canal discharges itself into a large vein situated near the heart, to the left of the base of the neck, called the subclavian vein of that side. 503. Right Lymphatic Trunk. — The lymphatic vessels of the right side of the body, including the arm, the chest, the neck and head, in like man- ner coalesce to form a canal, called the great right lymphatic trunk, which dis- charges its contents into the right sub- clavian vein. The contents of the lymphatic system being thus discharged into the two sub- clavian veins, are finally conducted, mingled with the venous blood, into the right auricle of the heart. 504. Lymphatic Glands. — A circum- stance of considerable importance is ob- served in the structure of the lymphatics, which requires special notice. In their course through the body to their points of junction with the venous system they are interrupted in various places by vascular masses of greater or lesser magnitude, into which they seem to discharge their con- tents. From opposite sides of these masses other vessels, generally of greater calibre, issue, which continue their courso. Be- tween the origin of the lymphatic system lymphatic ’otAND. and its points of junction with the venous, its course is thus 382 ANIMAL PHYSICS. frequently interrupted ; and in all cases where its continuance is resumed, the vessels acquire increased magnitude. Lymphatic Glands is the name given to these vascular masses, and the manner in which the lymphatics enter and leave them is illustrated in fig. 344, where 111 are the lymphatics entering, and 2 2 those issuing from them. These glands are generally of irregular roundish forms, and are found in various parts of the body, but more especially in the armpits, groins, neck, chest, and abdomen. 505. The thoracic duct, and the lymphatics and glands immediately connected with it, are shown in fig. 345, the parts being as follow : — 1. The thoracic duct passing in front of the vertebral column by tLe side of the azygos vein. Fig. 345. THORACIC DUCT AND GREAT LYMPHATIC TRUNK. (Mascagni./* * Vasorum lymph. Hist. P. Mascagni. Seuis, 17S". LYMPHATICS. 383 3. The origin of the thoracic duct at its confluence with the lymphatic vessels. These vessels are seen proceeding from the lymphatic glands of the abdomen, many of which appear at the lower part of the figure. 4. The point of confluence of the thoracic duct and left subclavian vein near the junction of the latter with the left jugular vein. 2. The large lymphatic vessels proceeding from the right side of the head, and discharging their contents into the right subclavian and jugular- veins. 506. It is impossible to contemplate the structure and dis- position of the lymphatics as here described without being struck with their analogy to the system by which a country is drained. The vessels in their most minute state of subdivision being scarcely more than microscopic, are distributed upon organs which are saturated with the fluid which it is their function to absorb. This fluid passes into them as the water which saturates a boggy soil passes into drains cut in various directions through it. These drains converging into a common channel, form a large stream; this stream, after pursuing its course for a distance more or less considerable, spreads out into a pool, where it absorbs fresh supplies. This is just what happens in the lymphatic system in passing from gland to gland. From this pool or lake a larger stream issues, which, converging with other streams, forms a considerable river; and thus tributary after tributary converging and coalescing, a vast river pours its waters into the sea ; just as the thoracic duct and great lymphatic trunk pom- the floods of nutrition which have been collected and absorbed through the system into the subclavian veins. The fluid mixture is conducted to the right auricle of the heart, whence, as described in the last chapter, it is conveyed through the right pulmonary arteries to the lungs, where it is exposed to the action of the oxygen of the atmosphere by a process which will be described in the next chapter. This last process is all that remains to reconvert the spoiled and used blood into fresh and nutritious arterial blood. 507. Contractile Action of the Lymphatics. — In the case of a river and its tributaries, to which we have compared the lymphatic system, the force which carries the fluid to the sea is gravitation acting along the descending beds of the several streams, and that of the principal river. In the animal economy, however, gravity, so far from aiding the march of the fluid through the vascular system, in most cases opposes it, the currents being directed upwards. In the case of the arteries, wo have seen that the moving 384 ANIMAL PHYSICS. force is the contractile action of the muscles of the heart, combined with the elastic power of the coats of the arteries, to which the valves of the heart supply points of reaction. It may then be asked, whether there is in the lymphatic system any propelling power, analogous to that of the heart. It is certain, that in mammifers, at least, there is no such organ, nor any which is analogous to it ; and, consequently, the cur- rent of the fluid can only be maintained by the contractile action of the lymphatics. 508. The Internal Structure of the Lymphatics indicates, if it do not fully prove, the existence of this contractile force. Like the veins, they are supplied with a series of semilunar valves, which prevent the reflux of the fluid, and supply points of reaction. One of the vessels laid open, so as to display the form and arrangement of these valves, is shown in fig. 346. 509. The membrane of the lymphatics is nearly transparent, and the lymph (453), with which most of them are filled, when not mixed with products of digestion, is also transparent and slightly yellowish in colour. When submitted to the microscope, colourless spherules are seen floating in it, which are smaller than the red corpuscles of the blood, and are, in fact, identical with the white corpuscles (445) seen float- ing in the blood. 510. Chyliferous Vessels. — The lymphatic ves- sels, which prevail in the region of the intestines, re- ceive the chyle (453), byabsorption, but do notdiffer in other respects from the lymphatic system. The lymphatics which thus conduct the chyle are some- times denominated Chyliferous or lacteal vessels. 511. Absorption by the Lymphatics is demon- strated by various easy and simple means. If the abdomen of an animal whose digestion is in full action be opened, the lymphatic vessels of the in- testines will be observed to be gorged with chyle, proceeding from the digested food. But, if the same vessels be examined in an animal after a long fast, they will be found empty and colourless. 512. The Movement of the Lymph and Chyle through the lymphatics is favoured by various anatomical and physiological conditions. Fig. 34G. SECTION OF A LYMPHATIC VESSEL SHOWING TnF. VALVES. LYMPHATICS. 385 It is easy to show that the transverse section of the thoracic duct, of the great lymphatic trunk, and of the vessels which immediately lead to them, is less than the sum of the transverse sections of all the lymphatic vessels which flow into them. The necessary consequence is, that in proportion as the passage open to the fluid is diminished, the velocity of the current is increased. This fact, however, though repre- sented by some physiologists as an active accelerating principle, cannot be so understood ; for, though the velocity of the fluid in the narrower passages is greater, the quantity which passes in a given time, and the propelling force, are absolutely the same. 513. The muscular action attending locomotion accelerates the progression of the lymph in the lymphatic vessels of the members. The contraction of the abdominal muscles produces the same effect on the progression of the chyle in the vessels of that region. 514. The mechanical phenomena of respiration, which will be more fully described hereafter, act in two ways, to favour the current of chyle and lymph towards the thoracic duct. The tendency to a vacuum, produced by the expansion of the thorax during each inspiration, not only causes the afflux of atmospheric air into the lungs, but also that of all the liquids which have access to the chest. Thus the liquid contained in the abdominal part of the thoracic duct, and in the nearest lymphatic vessels, is attracted towards its thoracic part during each inspiration. On the other hand, expiration, accompanied by the contraction of the abdomen, has a like effect, since it tends to make the liquid of the thoracic duct pass with more force from the abdominal to the pectoral region. 515. Structure of the Lymphatic Glands. — It was formerly supposed that in passing through the glands, the lymphatics entered into direct communication with the blood-vessels. The researches of modem physiologists have proved this to be an error. A lymphatic gland, as shown in fig. 344, consists of a mass of minute lymphatic vessels, among which numerous sanguiferous capillaries ramify. Between the two sets of vessels there is no inosculation. They conduct their respective fluids altogether independently of each other. The lymph which passes into the gland by the afferent vessels, passes out of it by the efferent ones, having in the gland been infinitely subdivided by the minute and multiplied tubes which form the 386 ANIMAL PHYSICS. substance of tlie gland. 'Whether there is any interchange between the blood of the capillaries in the gland by exudation or exosmose and the lymph of the smaller lymphatic vessels, Ls mere matter of conjecture, unsupported as yet by any results of immediate observation. 516. Sources of Lymph. — The liquid part of the blood, called the liquor sanguinis, or plasma, charged with nutritive principles, exudes by the process of exosmose through the coats of the capillaries, and being diffused among the tissues, supplies to them respectively the matters proper for their ! Radicles of the Chyle vessels. Intestine. Lymphatic Vessels. Mesentery. Fig. 347. CHYLE VESSELS OF THE MESENTERY. repair. The residuum of the plasma is absorbed by the multi- tude of lymphatics which pass through the same parts, into which it enters by the process of endosmose. In this state it constitutes lymph, and is carried back by the lymphatic vessels to the subclavian veins. LYMPHATICS. 387 It is probable that some interchange takes place in this case between the lymphatic vessels and the surrounding matter ; and that, while they receive the residuum of the plasma, they may give out to the tissues by exosmose some of their constituents. These are points, however, upon which much uncertainty still rests. It is certain, however, that the lymph is rendered coagulable by the fibrine which it receives from the plasma, more espe- cially in passing through the glands. Fig. 348. LYMPHATICS OF THE UPPER PART OP THE TRUNK AND HEAD (MaSCaglli). 517. Beautiful Structure of the Lymphatics.— There is no part of the organisation the structure of which presents a spectacle more curious and beautiful than the lymphatics. We c c 2 3S8 ANIMAL PHYSICS shall, therefore, give here some examples of their structure. In fig. 347 are shown the chyliferous vessels of the mesentery. These are spread over the intestines on the one side, whence Fig. 349. LYMPHATICS OF THE ABM AND hand (Mascagni). Fig. 350. LYMTHATICS OF THE LEG AND FOOT (Mascagni). they absorb the chyle, and, passing over the mesentery, are LYMPHATICS. 389 transmitted through a multitude of glands from which larger vessels issue, which eventually terminate in the thoracic duct. In fig. 348 are represented the lymphatics of the diaphragm, the heart, the breast, the armpit, the head, and the neck. In fig. 349 are shown the lymphatics of the arm and hand, and in fig. 350 those of the leg and foot. In figs. 319, 320, are shown the lymphatics of the heart. 518. The Lymphatics of all Vertebrate Animals of the inferior classes are similar to those of man. In the case of certain reptiles — the frog, for example — their structure is often more complicated than in warm-blooded animals. In the course of the lymphatic vessels of these are found certain enlargements, provided with muscular fibres, which have been called lymphatic hearts, whose contraction produces the same effect in propelling the lymph as the heart produces upon the circulation of the blood. In both reptiles and fishes the lymphatic vessels are relatively more voluminous than in mam- mifers or birds. Lymphatic glands, however, are generally absent in these classes. The valves are less numerous, and in some cases altogether absent. 519. In the larger class of Mammifers, the lymphatic and chyliferous vessels converge in a single thoracic duct, as in man. Frequently, however, this canal consists of two ducts, which remain separate up to the point where they enter the left subclavian vein. In other cases, although the thoracic duct is double in its pectoral part, and as far as the commencement of the cervical part, the two branches unite at the moment of joining the venous system. 520. The Lymphatic Vessels of Birds form by their union two thoracic ducts, which appear on each side of the base of the neck, uniting with the jugular veins. 521. The Lymphatics of Reptiles and Fishes terminate in the venous system by communications more or less numerous. The most frequent and largest of these communications are made with the veins in the immediate neighbourhood of the head. In mammifers generally, the lymphatic glands are numerous, and it is probable that they have no direct communication with the venous system. 522. The Invertebrate Classes havo neither chyliferous nor 390 ANIMAL PHYSICS. lymphatic vessels. There is no proper distinction between the blood and the product of digestion, or it may rather be said that this product constitutes the blood itself. In those which have a complete circulating apparatus with arteries distinct from the veins — such, for example, as the mollusca — it is pro- bable that the veins which circulate over the intestines absorb the products of digestion, and transfer them to the region of the respiratory organs. In arachnida, Crustacea, and annelida, whose apparatus of circulation is less complete, the product of digestion passes through the coats of the intestines, and is diffused through the regions which surround the digestive canal, and from thence, by imbibition and endosmose, is trans- mitted to the circulatory vessels. 523. In Insects the liquid product of digestion, after it has passed the coats of the digestive tube, does not pass into any circulating vessels properly so called ; it is merely diffused through the cellular interstices which exist among the organs, and thence into the organs themselves. 524. Radiata, excepting the echini and holothuria, have no vascular system, and the products of digestion pass through the sides of the digestive cavities directly into the tissues. The acalephas, which belong to this class, and which have the form of fungi, pi-esent a remarkable arrangement. The digestive cavity in these presents a multitude of parts, forming a com- plicated net-work, and the products of digestion escape through the sides of these minute reticulated intestinal tubes, their dispersion through the system being thus facilitated. INSPIRATION AND EXPIRATION. 391 CHAPTER VIII. RESPIRATION. 525. Respiration is the function by which venous is recon- verted into arterial blood. 526. The seat of this process is the lungs. 52 7. The change produced by it in the blood consists in imparting to it a portion of the oxygen of the atmospheric air which enters the lungs, and in extricating from it a nearly equal volume of carbonic acid. This interchange takes place through the membrane in which the pulmonary capillaries run, by the process exosmose and endosmose. * After this interchange the blood loses its venous character and becomes arterial. From a blackish-red colour it acquires a bright vermilion, regaining at the same time its nourishing property. In this state it returns to the left vessels of the heart, from whence it again passes into the circulation. 528. Inspiration and Expiration. — The air in the lungs being thus deprived of its due proportion of oxygen, and being charged with carbonic acid, is no longer capable of arterialising the blood ; it must therefore be expelled from the lungs and replaced by pure air. This is accomplished by the alternate contraction and expansion of the thorax. By its contraction the air is forced out through the windpipe and air-passages of the mouth and nose, and by its subsequent expansion fresh air is drawn in through the same passages. The contraction produces expiration, and the expansion inspiration. The play of this thoracic mechanism, and the consequent alternation of inspiration and expiration, never cease so long as life continues. The first act of the infant on entering the world is inspiration, and the last of the dying on leaving it expiration. So imperious is the necessity for the continuance of this process that its temporary suspension is attended with danger, and, if pro- longed beyond a certain limit, with death. 529. Respiration involves therefore two classes of phenomena, the one mechanical and physical, and the other chemical and physiological, which must be separately explained. * Hand-book of Nat. Phil. Hydrostatics, § 113. 392 ANIMAL PHYSICS. 530. Mechanism of Respiration. — The mechanism by which the alternate enlargement and contraction of the capacity of the thorax are produced consists in the peculiar structure of that bony cage, the muscles which act upon its moveable parts, and the motor nerves by which these muscles are excited. The thorax, as already explained (95), is a conoidal cage of a bee-hive form, the framework of which consists of the spinal column behind, the sternum or breastbone before, and the ribs at the sides. Relatively to the other parts the vertebral column is fixed ; the ribs are moveable, within certain limits, on their articulations with the vertebrae. They are moveable also rela- tively to the sternum, by reason of the flexibility of the carti- lages by which they are connected with it. In the state of repose which follows an expiration, the capacity of the chest being reduced to its least limit, each pair of ribs is inclined downwards and forwards, aud the sternum is depressed. To enlarge the chest during an inspiration each pair is drawn up to the horizontal position, and, at the same time, the sternum is elevated and protruded forwards. This movement produces an enlargement of the thorax by two of its dimensions ; firstly, by its increased depth from the sternum to the vertebral column, and secondly by its increased width from side to side. V Fig. S52. 531. Motion of Ribs and Sternum. — That such an enlarge- ment of capacity must ensue will be rendered apparent by the geometrical figures 351 and 352 ; the former being supposed to present a front, and the latter a side view of the chest. MECHANISM OF RESPIRATION. 393 The ribs, when in a state of repose and inclined downwards from the vertebral column, V V, are represented in fig. 351 by o A, o B, o C, o D, o E, o F, and o G. In fig. 352 the line S S is supposed to represent the sternum. The position of the ribs, when raised as already described, is shown by A', B', O', D', E', F'. In fig. 352 it will be seen that by this elevation of the ribs the sternum, S' S', is at the same time raised and thrown forward. It will be evident by a mere inspection of these figures that by this movement of the ribs upon their articulations, o, the transverse diameter of the thorax is augmented, A' A', B' B', &c. being obviously greater than A A, B B, &c. Thus the two dimensions of the thorax, one directed from the front to the back, and the other from side to side, are both increased. 532. Base of the Thoracic Cavity, the Diaphragm. — The spaces between the ribs are closed by the intercostal muscles. The base of the thorax is closed by the diaphragm, a muscle having nearly a hemispherical form, convex upwards and concave downwards, which is inserted in front in the sternum, behind in the vertebrae, and all round the sides in the ribs. This muscle is represented in the figures by the curved line A d d d d A, which divides the trunk into two compart- ments, the upper or pectoral included by the ribs, sternum, and vertebral column ; and the lower, or abdominal, included by the muscles and integuments of the abdomen and the lower part of the vertebral column. The pectoral compartment is appropriated to the apparatus of respiration and circulation, consisting of the heart, lungs, and their appendages ; and the abdominal to the apparatus of digestion, consisting of the stomach, fiver, intestines, and their appendages. 533. Respiratory Motion of Diaphragm. — When the ribs are raised in the manner here described, the diaphragm also undergoes a change. The mere elevation of the ribs would cause that muscle to become less convex by enlarging the area of its base ; but, independently of this, by its proper con- tractility it is rendered much less convex, assuming the form represented by the dotted fine A d! d' d' d' A. By tins means the height of the thorax, measured from its summit to the upper surface of the diaphragm, is augmented. How great an increased capacity can thus be given to the thorax by a comparatively small increase in each of its dimensions may be easily shown. Let us suppose, for example, that its depth, width, and height are each increased in the proportion of 4 to 5 ; in that case its capacity would be increased in the proportion of 4 x 4 x 4 to 5 x 5 x 5, 394 ANIMAL PHYSICS. that is, in the proportion of 64 to 125, or 2 to 1 very nearly. Thus an increase which would augment each of the dimensions in a proportion not greater than one-fourth would double its capacity. 534. Respiration illustrated by a Bellows. — Tin's alter- nate enlargement and diminution of the thorax by the elevation and depression of the ribs and diaphragm in respiration, are in all respects similar to the action of the common bellows {fig. 353). The ribs may be considered as analogous to the two boards A and B which open and close, their articulation with the spinal column to the hinges on which these boards play, and the diaphragm to the flexible leather by which the boards are united — -the nozzle, E, being the repre- sentative of the trachea or windpipe. When the ribs are elevated and the diaphragm extended and rendered less convex, the thorax represents the bellows with its boards separated. The enlargement of the capacity of the interior causes the external air to rush in through the windpipe to fill the vacuum which would thus be created. When the ribs are, on the contrary, depressed, and the diaphragm recovers its convexity, the internal cavity, being forcibly diminished, expels the air through the windpipe in the same manner exactly and upon the same mechanical principle as the air included in a bellows is expelled through the nozzle by forcing together its boards.* 535. Intercostal Muscles. — To render this operation effective, it is obvious that the thorax, like the bellows, must be air- tight. It is rendered so by the intercostal muscles and the diaphragm, which close all the passages both on the sides and at the base. The intercostal muscles consist of two layers superposed one upon the other, denominated the external and the internal. The fibres of each of these are arranged in a direction parallel to each other, and oblique to the ribs, — those of the external being nearly at right angles to those of the internal. 536. The Thorax with its Appendages is shown in fig. 354, the intercostals being removed from one side iu order to display the form of the diapluagm, the position of the parts * Hand-book of Hydrostatics and Pneumatics, 25G. MECHANISM OP RESPIRATION. 395 being that which they have in a state of repose, when the ribs are depressed after an expiration. It will be seen that the diaphragm ascends into the interior of the thorax to nearly half its height, and that its outline is not regularly convex, being somewhat depressed at the summit. Elevators of Ribs. Vertebral Column. Kibs. Intercostal Muscles. Clavicle. 3rd Rib. Sternum. Diaphragm. 7th Rib. False Ribs. V ertebrie. Pillars of Diaphragm. Fig. 354. THE THORAX WITH ITS PRINCIPAL MUSCULAR APPENDAGES. The external intercostals are shown on the left side of the figure, where the fibres are inclined obliquely from each rib downwards and forwards to the next below it. The internal intercostals are concealed by the external, but the position of their fibres may be easily understood by con- sidering that they are at right angles to those of the external intercostals, and are consequently inclined obliquely backwards from each rib to the rib below it. Some of the elevator muscles of the ribs are shown in the upper part of the figure, having their origin in the cervical vertebra), and their insertion in the upper ribs. The cartilages of the ribs are distinguished from the bony part by their light colour. The diaphragm is connected at its inferior surface with the lumbar vertebrae by muscles called its pillars or crura, and the ribs are severally connected with the vertebrae placed above them by sevoral smaller levator muscles. 396 ANIMAL PHYSICS. 537. Action of the Diaphragm in gentle Respiration. — ■ The respiratory action in man, when the body is not excited by exercise or labour, is performed by the elevation and depres- sion of the diaphragm, accompanied by a motion of the lower ribs. The enlargement of the thorax produced by the depression of the diaphragm is effected at the expense of the abdomen, the capacity of which is diminished as that of the thorax is enlarged. The cavity of the abdomen, however, being filled by the stomach, the intestines, and their appen- dages, the depression of the diaphragm acting upon these parts, combined with the reaction of the lumbar vertebrae, presses the contents of the abdomen forward, so as to protrude its anterior and lateral parts. When the diaphragm is again elevated by the act of expiration, the pressure of the atmo- sphere upon the external surface of the abdomen, combined with the elasticity of the integuments, forces the stomach and intestines backwards and upwards, so as again to fill the enlarged space under the diaphragm. 538. Such are the physical causes which produce the alter- nate heaving inwards and outwards, — or rather, the alternate swelling and contraction — of the abdomen which accompanies the act of respiration. 539. Pectoral Respiration in Females is greater than abdominal. This, which is the case naturally, is, however, greatly augmented by the effect of stays, especially when tightly laced. These, by resisting the natural play of the abdomen, obstruct the action of the diaphragm, and render necessary an augmented play of the pectoral mechanism. This exaggerated pectoral respiration is visible in the habitual heaving of the female bosom. 540. The intercostal muscles, having their origin in one pair of ribs and their insertion in the other, cannot act directly either as levators or depressors, from the want of fixed points of reaction : but when the ribs are rendered relatively fixed by the action of other muscles, then the con- tractile power of the intercostals comes into play. In fact the intercostals, properly speaking, ought to be regarded as the mere continuations of other muscles, never acting except in co-operation with them. 541. Respiratory Action of the Intercostals. Much dif- ference of opinion has prevailed among physiologists as to the MECHANISM OF RESPIRATION. 397 functions of the intercostals ; some considering the external as elevators, and the internal as depressors ; others, on the contrary, regarding the external as depressors, and the internal as elevators ; while others again have ascribed the same functions to both ; and, in fine, some insist that these muscles have no influence whatever on the movement of the ribs. The question, however, seems to be satisfactorily decided by a very simple geometrical exposition given by Hamberger,* which we here present with some modification. Let Y V, fig. 355, represent the vertebral column, B B, C C, D D, &c., the ribs when elevated ; and B B', C C', D D', &c., the ribs when depressed. Let a b represent a fibre of an external intercostal, and m n one of an internal. Let a' b' represent the same external fibre, and m' n' the same internal fibre, when the ribs are depressed. Now, it will be very easy to show by the most simple geometrical principles that a! b' is longer than a b, and m! n' shorter than m, n. To prove this, let b c, V c', m o, and in' o' be drawn severally parallel to Y V. It will then be evident that all these four lines will be equal in length to B C and D E, and therefore to each other. Now, that being the case, since B b is parallel to C a and B b' to C a', and since B b = B b', we shall have C c — C e subjected to asphyxia, or suffocated, by keeping the mouth forcibly open. The internal organisation of reptiles, including the parts engaged in respiration, will be illustrated by fig. 358, in which is shown that of the adder. 605. A temperature above 100° Fahr. is almost immediately destructive to most of them, and certain degrees of cold slacken all their vital phenomena. Most reptiles, therefore, losing their digestive power in winter, take no food. Their respiration is at the same time enfeebled in a remarkable manner, and they often fall into a lethargic state analogous to that of hibernating animals. 606. Aquatic Respiration is performed by organs which vary much in their form and structure. In some classes these consist of gills of foliated form, but in others are mere tubercles, having little more sensibility than the general integu- ment of the body. 607. The Respiration of Fishes is performed by branchiae or gills of foliated structure. These are highly vascular mem- branes attached to the external border of the branchial bones. In general, there are four branchiae on each side, each consist- ing of two rows of leaves. Most cartilaginous fishes, such as the ray, have five, and the lamprey has seven. With most of the bony fishes the gills are of simple structure, and fixed only at their base ; but with a few, among which may be mentioned the hippocampiis or sea-horse (fig. 359), they are ramified, and Fig. 359. THE HIPPOCAMPUS OR SEA-HORSE. have a resembance to feathers. In cartilaginous fishes they are generally attached to the skin at their external border, as well as to the branchial bones at their internal border. Respiration is accomplished, not, as in land animals, by air m RESPIRATION OP FISHES. 421 its free state, but by the portion of that fluid with which the water is always more or less impregnated. The water necessary for respiration entei's the mouth, and, by a species of degluti- tion, it is drawn in and driven through the interstices of the branchial bones. In this way it passes between the membranes forming the gills, which it washes, and then makes its exit by the orifices in which the gills are deposited. The animal is seen alternately to open its mouth for the reception of the water, and to raise the operculum of its gills for its discharge. In the case of fishes whose branchhe are not attached at their exterior edge, a single opening at each side suffices for the exit of the water ; but with those which have the external edge of the branchiae attached to the skin, as many openings are necessary as there are inter-branchial spaces. Thus, in the shark (fig. 360), there are five pairs of co-openings, and in the lamprey (fig. 361) seven. The internal arrangement Fig. 861. THE LAMPREY. of the respiratory apparatus can therefore be always inferred by the mere inspection of its external openings. With some species, the water does not pass directly from the mouth into the respiratory organ through the inter-branchial spaces, but arrives there by a canal, specially appropriated to that purpose, placed under the oesophagus, like the trachea in superior animals. 608. It will be evident, therefore, that fishes in general con- 422 ANIMAL PHYSICS. snme much less air in respiration than animals which live in the free atmosphere. There are some species, however, which, not content with the air dissolved in the water, rise from time to time to the surface to inhale the free atmosphere. Some swallow it, taking it into the intestinal canal, where the oxygen is converted into carbonic acid. The loach presents an example of this curious phenomenon. 609. When fishes are moved from then proper element, they soon perish from asphyxia. This arises, not from the want of oxygen, but because the gills, no longer sustained by the water, collapse and become dry, and are incapable of fulfilling their functions. It is accordingly found that the fishes which perish most promptly in the air are those whose opercula are very widely cloven, so as to facilitate the evaporation upon the surface of the gills ; while those which resist such exposure longest, either have narrow opercula or some internal vessel in which water is preserved to moisten these organs. The family of Labyrintliiform pharyngeals, of which the Anabas scandens, or climbing fish, and Asphromenus olfax, or gourami, are examples, are very remarkable in this respect, and owe their name to the water-cells which are placed above their gills. 610. These cells, as will be seen in fig. 362 included under the operculum and formed by the plates of the pharyngeal bones, effectually serve the purpose of retaining a certain quantity of water, which keeps the gills moistened while the animal is in the air, and enables it to continue there for a considerable interval without any suspension of its vital fimctions. These species are accordingly accustomed to issue from the rivers and ponds which are their ordinary habitation, and, gliding over the ground, to depart to considerable distances on their banks. The anabas has this labyrinthine apparatus in its highest degree of perfection, and not only remains a considerable time out of the water, but is even said to climb up trees. These species inhabit chiefly India, China, and the Moluccas. One Fig. 362. THE ANABAS. RESPIRATION' OF INVERTEBRATES. 423 species, called gourami, of Chinese origin, much esteemed for its flavour, has been acclimated in the ponds of the Isle of France and Cayenne. 611. The Respiration of Mollusca is aerial or aquatic, and performed in some by pulmonary, and in others by branchial organs. These organs are consequently subject to much varia- tion in form and position. 612. In Cephalopods the respiration is aquatic. The branchiae are symmetrical, and consist of leaves of arborescent form, much divided and subdivided, concealed by the mouth in a cavity having contractile sides. When it is dilated, water enters, and is expelled by its contraction. A cleft is provided for the entrance of the water, and a tunnel-formed tube for its exit. 613. In Gasteropods — those which have shells and have aerial res- piration— it is performed by means of a cavity, over the walls of which the pulmonary artery is ramified. This organ is generally placed in the last convolution of the shell. The air is admitted to the pulmonary cavity either by a small orifice left for the purpose in the shell, or by a canal placed between the body of the animal and the shell. Shelled gasteropods with aquatic respiration have branchiae. Sometimes the animal is obliged to protrude itself from the shell to put its branchial organ in contact with the water. Sometimes the respiratory organ is provided with a sort of caDal or siphon, by means of which it can be washed, the animal remaining in the shell. Nerines, volutes (fig. 221), cerites, porcelaines, whelks, are examples of these. Tecti- branchiate gasteropods have branchiae half concealed by the mantle. The nudi-branchiates, which are destitute of shell, have the branchiae on some part of the back. 614. The Molluscous Acephala are provided with four foliated and transversely striated branchiae, placed between the mantle and the body of the animal. 615. The Respiration of Insects is less perfectly localised, but it is aerial, the circulation being very imperfect. The blood is not animated with a complete movement of revolu- tion, and the air is conducted to meet it in its course at the same time in several parts of the system. The respi- ratory organ consists of a multitude of small tracheae, which communicate with the exterior of the body by openings called stigmata. The tracheae are sometimes merely ramified, but sometimes they have, from point to point, enlargements which form air sacs. They are kept open by a cartilaginous 424 ANIMAL PHYSICS. coating of spiroidal structure. The stigmata resemble little clefts or button-holes, which are sometimes furnished with small valves. There is generally one pair of stigmata for each ring of the body. The renewal of the air in the trache* is produced by the alternate expansion and contraction of the abdomen. Respiration in winged insects is somewhat active, and, as a consecjuence, their temperature is sometimes elevated in a remarkable degree. Head. 616. Anatomy of the Nepa. — The respiratory apparatus of insects may be illustrated by a reference to the general structure of the nepa, in- cluding that apparatus shown in fig. 363. RESPIRATION OF INVERTEBRATES. 425 617. The Respiration of Arachnida, like that of insects, is aerial, and performed sometimes by the tracheae and sometimes by air sacs placed in the abdomen, which as much resemble branchiae as lungs. They present in their interior a multitude of lamellae, resembling the foliated structure of gills. They receive air, like the tracheae, by means of stigmata placed along the sides, or upon the lower surface of the abdomen. 618. The Respiration of Annelidas is generally aquatic, branchiae being subject to much variation both in form and position. Sometimes — as, for example, in the arenieola or sand-worm, fig. 364 — they form tufts placed at equal dis- tances along the body. Sometimes, as in the nereids, fig. 208, they are grouped round the foot in the form of tubercles, and sometimes they are placed at the extremity of the body in a feather-like form, of which serpulse, fig. 210, present an example. The only annelids which have not aquatic respiration are the common earth-worms, which live in the humid soil, and have cutaneous respiration, com- bined, perhaps, with respiration by small air sacs placed in the anterior part of the body, and communicating with the external air by pores. 619. The Respiration of Crustacea is in gene- ral aquatic and performed by branchiae. Some of this class, however, having no branchiae, respire by parts of the body covered by a soft integument. Some Crustacea which live in the air respire by means of a multitude of external lamellae kept in a state of permanent humidity, which form branchiae and perform the office of lungs. 620. The Respiration of Zoophytes in general is performed without special organs, the inter- change of gases with the atmosphere being made by the tegumentary covering of the body, as well external as internal. In the case of some — as, for example, the holotliuria, fig. 229, a special ramified canal is provided, somewhat ana- logous to a trachea, into which the water is introduced by a cloaca, and expelled from time to time by the contraction of the sides of the canal. In the case of infusoria , vibra- tile cilia are observed upon the surface of the body, by the movement of which the water of respiration is renewed. Fig. 304. AREN1COLA OR SAND-WORM. 426 ANIMAL PHYSICS CHAPTER IX. DIGESTION. 621. Waste of the Body. — Every motion, whether volun- tary or involuntary, of the body or its organs, is attended with a certain wear of its structure. The parts worn out are dis- missed just as are those of a piece of mechanism abraded by friction. This constant wear would at length produce decay and dissolution, if means were not furnished by which the organised matter lost could be restored. 622. Repair by Nutrition. — Nature has therefore provided means by which the animal, taking from external organised bodies parts of their substances, appropriates them to the repair of its own body. The function by which this is accomplished is called nutrition. 623. Digestion. — The matter thus received not being in a state suited for nutrition, and containing, moreover, certain constituents which cannot be at all adapted to that purpose, it is submitted in the organism to certain operations by which certain parts are modified so as to render them fit for incorpo- ration with the system, and the residuum incapable of such modification is separated and expelled. This process is called digestion. 624. Absorption. — When the part fitted to replace the waste has been thus prepared, means must be provided by which it can be transported into and duly incorporated with the blood, which, as has been already explained, is the imme- diate agent by which the body and its organs are nourished. This is accomplished by the transmission of the matter pre- viously prepared by digestion into the torrent of the circulation, either through the intervention of the lymphatics, or more directly through the coats of the veins and other vessels. This process is called absorption. 625. Alimentary Canal. — Vegetables receive nutriment into APPETITE AND HUNGER. 427 their tissues directly from the earth by their roots, and from the air by their foliage. The nutrition of animals is more complicated, the matter from which it is derived not being so immediately suited for absorption. They are accordingly fur- nished with an internal apparatus, consisting of a cavity in which the matter taken from external organised bodies is elabo- rated, the nutritious parts being absorbed, and the unnutritious residuum expelled. In man and the superior animals this digestive cavity has the form of a tube open at both ends, of considerable length and variable diameter. It is called the alimentary canal. 626. The Phenomena of Digestion are therefore mechanical, chemical, and physiological. The mechanical phenomena consist in the prehension of the aliments, their mastication, deglutition, propulsion through the alimentary canal, and in the expulsion of their undigested residuum. The chemical phenomena consist in the decomposition of tho aliment, the separation of its nutritious from its unnutritious constituents, and the solution of the former by, and their com- bination with, the several juices secreted in the digestive canal, so as to form a fluid suitable to nutrition. The physical phenomena consist in the transmission of this fluid, through the coats of the vascular system, into the blood by absorption. 627. Appetite and Hunger. — The Author of nature, in his infinite beneficence, has appointed pain as a notice of approach- ing injury or bodily derangement. In proportion as the injiuy becomes nearer and more grave, the warning becomes more urgent. Nothing can be more admirable than the degrees by which this internal notice is regulated. The want of food in due time is attended at its first approach, not with positive pain, but with a desire, or appetite, as it is called, which holds an intermediate place between pleasure and pain — the pleasure consisting in the anticipation of gratification, and the pain in a slight degree of uneasiness attending the momentary postpone- ment of this satisfaction. If the supply of food, however, be postponed so long as to produce the commencement of injury uneasiness ensues, which increasing, becomes painful, and receives the name of hunger. 628. The commencement of appetite coincides with the 428 ANIMAL PHYSICS. completion of the digestion of the food previously taken into the system. Its periodic returns vary with age, hygienic con- dition, and bodily habit. It may be stated however, generally, that the desire for food will recur with a frequency and inten- sity proportional to the activity and rapidity of digestion. Thus children are sensible of hunger more frequently than adults, convalescents more so than those who enjoy uninter- rupted health. This is easily explained. Children and con- valescents require food not only to replace the normal loss sustained by then organs, but to supply the means of in- creasing the quantity of matter in their system — the one because they grow, and the other because they require to regain that which was previously lost in the abnormal con- dition produced by disease. Exercise and labour also develop the sense of hunger, •while sedentary habits have a contrary effect ; because the one stimulates digestion, while the other retards it. 629. Hunger, which is renewed in man two or three times a-day, is more imperious in animals of more active circulation and higher temperature. Thus, birds cannot survive a fast of twenty- four hours. Those animals, on the contrary, whose circulation is less active and temperature lower, feel it less frequently. Thus, hibernating animals, and some reptiles, can sometimes remain months without nourishment. It is said that the leech requires a year to digest the blood with which it is gorged. 630. The temperature of the medium in which an animal lives has a marked influence upon appetite. A low temperature stimulates hunger, while an elevated one renders it languid. In cold climates man struggles against the external cold by taking aliments in quantity and quality adapted to increase those internal combustions which are the source of animal heat. 631. Though the want of food be not so immediately destructive of life as the want of air, its ultimate effects are not less so. Animals deprived of it suffer a gradual decrease of weight and strength, and if the want be continued, death supervenes, after sufferings more or less prolonged. 632. Thirst. — Every cause which diminishes the due propor- tion of the aqueous constituent of the economy awakens the sense of thirst. An elevated temperature, by stimulating cuta- neous and pulmonary evaporation, and violent exercise, which CONSTITUENTS OP ALIMENT. 429 increases the secretion of sweat, have this effect. Thirst is also excited in a morbid degree by that class of maladies which are attended by an undue secretion of urine, or by abundant haemorrhage. The sense of extreme thirst is even more intolerable than that of hunger. Shipwrecked persons suffer more from priva- tion of drink than from privation of food ; and death from the former cause is more rapid than from the latter. Salt aliments produce thirst, because their saline constituents require a super- abundant supply of liquid juices for their solution in the alimentary canal, and thus diminish the proportion of water in the blood. Substances which irritate the stomach, such as pepper and spice, have a like effect. Since the sensation of thirst arises from an undue diminution of the aqueous constituent of the blood, any expedient which will restore the aqueous element, even though it do not act through the stomach, will diminish or remove the sensation. Thus, shipwrecked persons deprived of drink are enabled to slake their thirst to a certain degree, and prolong their lives, by immersion from time to time in the sea, retaining round their bodies the sea water with which their vestments are saturated. The water with which they are thus surrounded is decomposed by evaporation, the salt being dis- missed, and a portion of the pure fresh water entering the blood through the pores of the skin. 633. Aliment — its Constituents. — Since the food taken into the system supplies the sole materials out of which the body is formed and maintained, it must necessarily contain all the con- stituents of the body in a due proportion. The food used by man is exclusively animal or vegetable ; but the meat, vege- tables and fruits which we eat, the water, wine, and the liquors which we drink, include, besides their proper organic principles, certain others, such as salt, lime, sulphur, phosphorus, iron, and other mineral substances. These mineral constituents, like the organic matter properly so called, are appropriated to the renewal of the solid and liquid parts of the organism ; for the tissues eontain all these several mineral constituents. Among the substances thus derived from the mineral kingdom, salt plays the most important part in the functions of digestion, by favouring the secretion of the juices, exciting the sense of thirst, and the introduction of liquid, which aids the process of absorption. All the superior animals as well as man have 430 ANIMAL PHYSICS. therefore a marked appetite for this mineral, and eat it with avidity. 634. Mineral Substances alone cannot support Life. — Although mineral substances thus play an important part in the phenomena of digestion, they are incapable of themselves to support life. Nomadic and savage tribes, to appease the sense of hunger, sometimes introduce into the stomach certain aro- matic earths, but never derive nutrition from them, except when they contain some organic principles. If organic sub- stances, on the contrary, are found to be sufficient for the maintenance of life, it is because independently of their organic constituents, properly so called, they also include the mineral matter necessary for the supply of the tissues. 635. Animals denominated Herbivorous, Carnivorous, and Omnivorous. — Although all the constituents of food, such as charcoal, lime, salts, and gases, are found in the material world in their simple state in unlimited quantities, the animal organs are not so constituted as to appropriate them immediately, and to convert them by mere digestion into a state suitable for their maintenance. These simple alimentary principles must undergo a previous process, by which they are transmuted from the unorganised to the organised state before they are fitted for animal nutriment. This change is effected chiefly in the vege- table kingdom, where such substances are converted into those forms of organised matter which constitute vegetable food. This forms the exclusive aliment of a certain class of animals, which are thence denominated herbivorous. Other animals are provided with a digestive apparatus which is not capable of converting such forms of food into the organised matter of their bodies, and these feed upon the bodies of other animals which are destined by nature to become their prey. Such are accordingly denominated carni- vorous. In fine, there are other classes, among which man is included, whose digestive .apparatus is adapted to both binds of aliments, and these are accordingly denominated omnivorous. 636. Nitrogeniscd and Non-Nitrogenised Aliments. — Not- withstanding the infinite variety of form and quality of which food is susceptible, it may be reduced to two classes, having an NITROGENISED FOOD. 431 important relation to tie digestive functions ; one of which is characterised by tie presence, and tie other by the absence, of azote or nitrogen as a constituent. The one class is accordingly denominated nitrogenised, and the other non-nitrogenised aliments. All nitrogenised aliments include a quarternary compound, consisting of carbon, hydrogen, oxygen, and azote ; and all non- nitrogenised aliments include a ternary compound, consisting of carbon, hydrogen, and oxygen, with the exclusion of azote. Both of these combinations enter into the composition of all food, animal and vegetable, but the nitrogenised character has a marked predominance in the former, and the non-nitrogenised in the latter. 637. The combination of these two principles in due propor- tion is indispensable to the nutritious character of all aliments. Each of the two principles is endowed with its peculiar physio- logical property. The nitrogenised part goes to the formation and repair of the tissues, and the non-nitrogenised to the pro- duction of that animal heat which is necessary to the main- tenance of the temperature of the organism. Nitrogenised food has therefore been sometimes denominated plastic, and non-nitrogenised, respiratory or calorifacient. Examples of nitrogenised aliments are presented in food of animal origin; by the lean of meat, and, in general, the muscular parts of all flesh, the white of eggs, milk, and cheese, and the gelatinous principle extracted in soup from various parts of the animal, such as the tendons, ligaments, membranes, skin, and bones. Similar examples are presented in the case of vegetable food by the gluten, which forms the nutritive principle of most kinds of grain, and many seeds ; the albumen of all vegetable juices, and the caseine which constitutes the nutritious principle of peas, beans, lentils, and the like. Examples of non-nitrogenised aliments are presented in the case of food of animal origin, by fat, butter, milk, and honey ; and in food of vege- table origin, by starch, sugar, gum, mucilage, and the gelatinous principle of fruits and oils. 638. The imperious necessity for nitrogenised constituents in food is demonstrated by the fact that nitrogen constitutes an im- portant part of the animal tissues ; and since, in the phenomena of digestion, no elementary substances can be evolved except such as are already contained in the food, it follows that the presence of nitrogen as a constituent of food is indispensable. Plants, it is true, take an important part of their nourishment from the atmosphere ; but man, instead of borrowing from the atmosphere, gives to it, in respiration, a large quantity of carbon, and at least as much azote as he receives from it. 432 ANIMAL PHYSICS. 639. Since nitrogenised and non-nitrogenised matter.-; enter into the composition of animal as well as vegetable food, it follows that man can subsist on the one or the other ; but, since there is a deficiency of the nitrogenised principle in the one, and of the non-nitrogenised in the other, either regimen adopted exclusively will be inferior in its nutritive power to one in which both are duly mixed. Haller ascertained, by numerous and well-conducted experiments, that an exclusively vegetable regimen was productive of a diminished development of the tissues, and enfeebled muscular power. The operatives employed in the iron works of Tarn, in France, had for a long time subsisted on a regimen exclusively vegetable. It was observed that during this period each of them was disabled by fatigue or indisposi- tion an average number of fifteen days per annum. In 1833, M. Talabot, taking the direction of the works, changed the regimen, adopting the ordinary one of animal and vegetable food. The health of the men was so increased^ that after this the number of days per annum lost by indisposition was reduced to three. The mixed regimen therefore gave twelve days per head per annum of greater capability of labour. 640. The difference between animal and vegetable food is one therefore of proportion only, and not of kind, both con- taining the nitrogenised and non-nitrogenised constituents, though in different proportions. It is for this reason that animals entirely carnivorous can be nourished artificially on vegetable food, and those entirely herbivorous on animal food. The pig, which fives entirely on roots, can five upon meat, and the dog, naturally carnivorous, on bread. The proportion of nitrogenised constituents contained in vegetables being comparatively inconsiderable, herbivorous animals supply the deficiency of nitrogenised matter by the increased quantity of food consumed. A horse or an ox consumes per day the tenth or twelfth of their weight in vegetable food, while the dog or cat are able to subsist upon the thirtieth of their weight in animal food. It is for this reason that the digestive canal of herbivorous is formed with greater capacity than that of car- nivorous animals. 641. In the human organism there are many indications of the omnivorous character. The dentary mechanism includes the incisors and canines of the carnivorous, and the molars of the herbivorous animals. The capacity of the digestive canal exceeds that of the carnivorous, while it falls short of that of the herbivorous animals. Experiments prove that the exclusive use either of nitrogenised or non- nitrogenised food is incompatible with the continuance of life. Thus, dogs fed with sugar, olive oil, gum, or butter, by M. Magendie, lost weight, and PREHENSION OF FOOD. 433 lived only thirty days. Geese fed upon sugar, gum, or starch, by Messrs. Tiedemann and Gmelin, lost weight and died from the sixteenth to the twenty-seventh day. Like results followed from the exclusive use of nitrogenised food. A goose fed by Messrs. Tiedemann and Gmelin upon the white of egg, boiled and hashed, died upon the forty-sixth day. Dogs fed exclusively upon fibrine, albumen, and gelatine, either sepa- rately or together, survived for three months only. 642. When the aliment on which the animal subsists exclu- sively contains nitrogenised and non-nitrogenised constituents in suitable proportions, life and health can be sustained by it. Thus, milk given exclusively can support life; bones will nourish dogs. Ilice alone has also supported animal life, because it contains a considerable proportion of nitrogenised constituents. Peas, lentils, and beans, which contain the same proportion of nitrogen, have probably a corresponding nutritious effect. 643. Independently of the peculiar composition of each kind of aliment, variety is found to be indispensable to the health of the animal. Thus, if we take an animal which feeds indifferently upon various sorts of vegetables — a rabbit for example, which eats several kinds of grain and vegetables, such as cabbage and carrots — it will fatten, if supplied with these several sorts of food together or in succession ; but if it be limited to any one of them it will exhibit all the external signs of starvation, and in two or three weeks will die in the same manner as it would of total want of food. 644. Man rarely consumes aliments, whether animal or vege- table, in the state in which nature supplies them. They are generally submitted to a previous culinary preparation, by which their digestion is rendered more easy. This consists in combining together different alimentary substances so as to transform incomplete and imperfect into more perfect food. It is thus that vegetables which contain but little azote are mixed ■with the gravy of meat and milk, which impart to them more nutri- tive properties. The different condiments which are taken with food, such as pepper, salt, mustard, and the exciting liquids which are used with these seasonings, such as lemon, vinegar, &c., act upon the stomach in a manner to favour secretion of the gastric juice, or more directly to co- operate with that principle. 645. Prehension of Food.— Nature has provided all animals with instruments variously adapted to the seizure of the food and its conveyance into the alimentary canal. In man, the superior members, composed of the hands and arms, accomplish this. The joints of the arm are so disposed as to direct the 434 ANIMAL PHYSICS. hand to the mouth with the greatest promptitude and facility ; and the articulation of the head with the trank facilitates at the same time its inclination to meet the hand. 646. When the aliment has a volume disproportionate to the magnitude of the buccal cavity, into which it must be introduced, we divide it either by means of the hand, or by the intervention of mechanical instruments. Sometimes the teeth are used for this purpose, one part of the food to be divided being seized by the incisors, while another is drawn from the mouth by the hands. Man also, like the inferior animals, can seize his food directly with the mouth, without the intervention of the hand, but the prominence of his nose and chin render this operation much more difficult than in the case of animals, in which the mouth, by its form and position, is more expressly adapted for prehension. Man, therefore, never uses the mouth for prehension, save in the exceptional cases where he is de- prived of the use of Iris hands and arms. fid1/. Prehension of liquids. — Liquid nourishment is gen- erally drawn into the alimentary canal by the process of suction, which is the first nutritive act of the infant on coming into the world. The action of the infant mouth in drawing milk from the maternal pap is a curious physical experiment. The lips are applied to the breast round the pap so as to be in air- tight contact with it ; at the same time the tongue, which pre- viously fills the mouth, is withdrawn, the veil of the palate being closed so as to render the interior of the mouth a partial vacuum. The atmospheric pressure then taking effect upon that part of the flesh of the breast which is outside the infant’s bps, acts upon it so as to press the milk through the opening of the teat into the mouth. When the mouth is thus filled the veil is opened and deglutition takes place, the milk descend- ing through the upper part of the alimentary canal into the stomach. The veil of the palate is then again closed, and the same operation repeated. In this case the mouth acts like a common India-rubber syringe, the sides of which, being first pressed together, exclude the air ; and when allowed to expand, the liquid in which the syringe is immersed is forced hi by the atmospheric pressure acting upon its external surface. The tongue also plays the part of the piston of a common syringe, first filling the mouth so as to exclude the air, and then, by being withdrawn, leaving above it a partial vacuum, so as to give effect to the external pressure of the atmosphere. When a man drinks from a cup MASTICATION. 435 or glass the operation is performed upon the same principle ; the under lip being applied to the exterior part of the edge of the cup in air-tight contact with it, and the upper lip being at the same time immersed in the liquid, a partial vacuum is pro- duced within the mouth in the manner explained above, and the atmospheric pressure acting upon the surface of the liquid in the cup forces it into the mouth. In some cases the liquid is drawn into the mouth without the direct intervention of atmospheric pressure. Thus, when we take soup from a spoon, without placing the upper lip in contact with it, a current of air is drawn into the mouth between the lip and the liquid. This current, acting upon the surface of the liquid, has a tendency, as it were, to blow it into the mouth. In such a case, however, the movement of the liquid is chiefly produced by its gravity, the outer part of the spoon being a little lifted, so that the liquid falls, as it were, into the mouth. 648. Mastication. — The adaptation of the jaws and teeth to the division and trituration of food has been so fully de- scribed in a former chapter, that little need be said of it here. The subdivision of the food by mastication is essential to its easy digestion, by exposing a larger surface of it to the action of the juices secreted in the alimentary canal. This has been proved directly by experiments on artificial digestion, in which food more or less broken up and triturated has been exposed to the action of these juices. It is, however, more especially with vegetable food that mastication is indispensable. The nutritive parts of vegetable food generally are included in envelopes or husks, which would resist the digestive juices. These envelopes must therefore be broken by the teeth, to extract the alimentary matter. Animals which live on vege- table diet, such as grain and forage, masticate their food much more perfectly than do carnivorous animals, whose masticating apparatus is adapted rather to seize and tear their prey than to triturate it. Aged horses, whose teeth are worn, often perish by reason of the iudigestibility of their nourishment, when care is not taken to have it chopped and bruised. 649. The process of cooking, to which vegetable food for man is subjected, contributes to render mastication more easy by softening and even bursting the envelopes which include the nutritive matter. Hut even then mastication is still necessary. When it is not effected, vegetable food, such as peas, beans, p f 2 436 ANIMAL PHYSICS. ;incl lentils, often passes through the system, and is rejected unchanged in the f feces. How much the efficacy of the digestive functions depend on due mastication is well understood by those who suffer from imperfect digestion. 650. Alimentary Canal. — Complicated as is the series of physical and physiological processes which are carried on in the digestive apparatus, it is surprising by what simple means nature has attained her purpose. To render the form and structure of the alimentary canal more easily intelligible, we shall first consider it to be extended into a straight line, post- poning for the moment the explanation of the manner in which its vast length is packed into the apparently small capacity of the abdominal cavity. Supposing then the entire digestive canal to be extended in one continuous straight line, commencing at the mouth, where the food is introduced, and terminating at the anus, where its undigested residuum is expelled, the whole apparatus thus arranged for illustration is represented in Fig. 365, the several parts of which it consists, and appendages annexed to it, being marked upon it. Its total length in the human economy, from the mouth to the lower extremity, is about thirty feet. It is a flexible membranous tube of small calibre. In the figure, the several parts of its length are represented nearly in the proper proportion upon the scale of an inch to two feet. The diameter, however, of the different parts is necessarily on a larger scale. To bring the figure conveniently within the page, the entire length is divided into two equal parts, the top of that which is at the right of the page being the continuation of the bottom of that which is at the left. It will be observed that the tube is not of uniform diameter, but has several enlargements, as well as appendages passing into it by lateral tubes. The first enlargement of its summit is the mouth, separated from the second, called the pharynx, by a contracted aperture furnished with a valve open- ing backwards and downwards, through which the food passes. The pharynx is succeeded by a portion of the tube about nine inches in length, and less than one inch in internal diameter, called the (esophagus, which terminates in another enlargement of this cavity, which, when completely filled, contains from three to four pints. This membranous bag is the stomach. From the stomach the canal is continued by a narrow flexible tube about twenty feet long, called the small intestine. Tliis tube is divided by anatomists into three parts of unequal length, — the first issuing from the stomach, and having a length of about nine inches, and called the duodenum, from the circumstance of its estimated length being twelve finger breadths. The diameter of this part varies from an inch-nnd-a-haif to an inch and three-quarters. This is followed by the jejunum, measuring about eight feet, and having a diameter which varies from an inch to an inch-and-a-half. This part of the tube derives its name from the fact of Mouth PoncreaCs Liv<»r (!«cum Rectum 7 in. M| Ph.irynx ! 1 Oesophagus Stomach p i ' mtmm ¥r y t;,n Sigmoid 1* it? . 30c • THEORETICAL DIAGRAM OF THE ALIMENTARY CANAL IN THE HUMAN ORGANISM. 438 ANIMAL PHYSICS. its being found generally empty after death. The last and largest division of the tube, measuring about twelve feet in length, and having a diameter varying from an inch to an inch-and-a-quarter, is called the ileum, — a Latin word signifying a small gut. The ileum is succeeded by part of the digestive canal, measuring five feet in length, and about two inches and a half in diameter, called the large intestine, the structure of which is altogether different from that of the small intestine. As shown in the figure, it consists of a series of annular contractions, and having a muscular structure, it hasthe property of pushing onwards the residuum of the food which passes through it towards the extre- mity, where this annular structure ceases, the tube becoming again cylin- drical. The terminal part of the alimentary tube which has this cylindrical form, or nearly so, is called the rectum, and is about seven inches in length, having a diameter somewhat less than that of the sigmoid flexure which enters it ; but it becomes dilated into a large ampulla, or reservoir, for the reception and accumulation of the feces immediately above the anus. This terminal enlargement is not represented in the figure. The extremity of the large intestine into which the ileum enters is called the ccecum, from being a blind pouch, or cul de sac, and the other ex- tremity, which opens into the rectum, is called, from a circumstance which will presently be explained, the sigmoid flexure — the intermediate part of the great intestine being called the colon. The opening from the stomach into the duodenum, called the pylorus. Base of skull Pharynx (Esophagus Tongue. Salivary glands. Hyoid bone. Larynx. Thyroid gland. Trachea. Fig. 36(1. PHARYNX and (KSOPHAGUS. (Edwards.) is surrounded by a strong muscle, capable of closing and opening, so as to perform the function of a valve. The piece of mechanism takes its name ALIMENTARY CANAL, 439 from a Greek compound — ttuAtj (pnl6), a door, and oBpos (ouros), a guard. A valve, called the ileo-cacal valve, is also interposed between the ileum and the caecum. 2 Lower end of (esopha- gus. 1 Stomach. 3 Left end of stomach. 4 Right end of stomach. 5, 6 Duodenum. 7 Convolutions of jeju- num. 11, 12, and 13 Ascending, transverse, and descend- ing colon. 8 Convolutions of ileum. 9 Caecum. 10 Its vermiform ap- pendix. 14 Sigmoid flexure. 15 Rectum. Fig. 367. The ducts of the liver and gall-bladder, and of the pancreas, — organs ■which will be described hereafter — open into the duodenum a little below the pylorus. 440 . ANIMAL PHYSICS. (351. Let us now consider liow the comparatively enormous length of this apparatus is not only packed within the limited capacity of the abdomen, but so arranged there that the function of digestion performed within it proceeds with regu- larity during the whole continuance of life. The relative position of the mouth, the pharynx, and the oesophagus, L-- represented in fig. 366, in which a vertical section of these parts is shown. -- Colon. Gall-bladder Spleen. Large intestine — Small intestine. ... Colon. Caecum Vermiform ap-1 pendix of the [• ca»cum. j Small intestine. Rectum. Fig. 36S. AMMEXTARV CANAL. (Edwards.) The oesophagus, whioh takes its name from a Greek compound signifying STOMACH. 441 food-bearer, is a membranous tube which passes down the neck be- hind the windpipe, between the two carotid arteries, and descending through the thorax, passes through the diaphragm, and enters the upper part of the stomach. The stomach, and all the inferior part of the alimentary canal, are represented as naturally arranged within the abdomen in fig. 367. It will be seen that the stomach is an oblong bag placed horizontally, the cesophagus (2) entering at the uppermost portion, and a little on the left of the middle of the cavity. The duodenum (5, 6)'. eads from the right side of the stomach, and bending downwards, descends to the jejunum, the convolutions of which occupy the upper part of the abdominal cavity. Below these are the con- volutions of the ileum, the lower extremity of which enters the large intestine laterally, — that small portion of the great intestine (9) below the point of junction with the ileum, being called the cseeum. The great intestine rises so as to form a sort of arch, the upper part of which is nearly in contact with the lower part of the stomach. From the form in which it is arranged in the abdomen, the colon is distinguished into three parts, called the ascending, transverse, and descending parts. The sigmoid flexure (14) takes its name from the peculiar form into which the intestinal canal is there bent, bearing a resemblance to the Greek letter 2. The terminal ampulla of the rectum (15) already mentioned, is shown in the figure. The liver and pancreas, which are not represented in fig. 367, are included in fig. 368, the several parts being indicated as before. 652. Coats of Alimentary Canal. — The alimentary canal, from the pharynx to its lowest extremity, is lined throughout with a covering called the mucous membrane, which, although a continuation of the skin, differs from it by the absence of the epidermis, which is replaced by a soft cuticular tissue, called the epithelium, thickly overspread with a network of minute blood- vessels and lymphatics, and containing innumerable secreting pores. This membrane is sheathed in another of muscular structure, by the contractions and relaxations of which the contents can be moved in the canal during the process of digestion, or arrested at certain points during intervals more or less continued. In fine, a third coating, consisting of a serous membrane, called the peritoneum, encloses the whole. A fourth coating is also described, composed of cellular or areolar sub- stance, interposed between the mucous membrane and the muscular coat. 653. The Stomach.— The stomach, into which the food, after mastication and insalivation, is transferred through the cesophagus, is a bag, lined by an extensive and loosely connected mucous membrane, which, when empty or only partially filled. 442 ANIMAL PHYSICS. forms wrinkles and folds on its internal surface. These, however, disappear altogether when it is distended by food. The narrow opening at the lower extremity of the oesophagus, at which the food enters the stomach, is called the cardiac orifice.. After the passage of the food this orifice is closed, so as to resist its return towards the mouth, which otherwise might take place by the action of the contractile force of the muscles surrounding the stomach. Sometimes, nevertheless, more or less of the food is forced upwards and thrown into the mouth, as in the case of eructation or vomiting ; this happens, however, with more facility, and therefore with more frequency, in the case of gases evolved in the stomach. 654. The Mechanical Action of the Stomach forms an important part in the process of digestion. By this involuntary action the aliment is rolled about, so that every part of it is brought successively into contact with the mucous membrane which lines the stomach upon which the gastric juice is secreted. Every part of the food is thus successively impregnated with the gastric juice. The necessity of this stomachic action is illustrated by dividing the pneumo-gastiic nerves (fig. 270, 270, 13), which excite this involuntary motion. After their section the motion ceases, and no parts of the food receive directly the gastric juice except those which lie on the outside of the mass and immediately in contact 'with the coats of the organ. The central parts consequently remain undigested. This mechanical action of the membrane of the stomach has been demonstrated in various ways. M. Reclam, having kept a dog fasting for a sufficient time to render the stomach empty, gave it an abundant meal of milk very rich in caseine, which consequently coagulated upon being received into the stomach. Having opened the animal, and removed the stomach, he showed the marks of its mechanical action upon the coagulated mass. The same physiologist produced a series of artificial digestions by placing broken and triturated food in phials surrounded with gastric juice, and maintained at the temperature of the blood of the living animal. When the phials were kept at rest, the digestion of the food was only superficial, but when they were agitated it was perfect. In the case of patients suffering under gastric fistula, rods of whalebone introduced through the fistula into the interior of the stomach were sensibly affected by the internal movements. These movements have been also felt directly by the finger introduced through the fistula of a dog and brought into immediate contact with the stomach. 655. Tlie stomachic movements arc not simultaneous, but successive, passing over the surface of the organ with a sort of MOVEMENTS OF THE STOMACH. 443 undulatory motion, the effect of which is obviously to transfer the food successively from one part of the stomachic cavity to another, and thus to mix it more effectually with the gastric juice. The character of these movements varies in different animals, according to the modes of digestion. M. Schultze showed that in the case of herbivora it is a motion of revo- lution, while in that of carnivora it is an alternate motion from side to side. Dr. Beaumont observed it in a patient affected with gastric fistula, and found that it was similar to that which M. Schultze had ascertained it to be in the case of herbivora. The aliments made a complete revolution of the stomach in from one to three minutes. When the stomach is paralysed, as above explained, by the section of the pneumo-gastric nerves, it may be put in action by the irritation of the extremity the nerve which leads to it, in which case the efficacy of the digestive process is artificially restored. 656. Eructation. — When gases are evolved in the stomach they produce an uneasy, and often a painful sensation. They are sometimes expelled through the oesophagus by the mere contraction of the stomach, or by that contraction aided by the abdominal muscles and the diaphragm. By them specific levity they have always a tendency to accumulate in the most elevated part of the stomachic cavity ; and therefore, when we stand or sit upright, they are collected immediately under the cardiac orifice, and then- expulsion is rendered more easy than when the body is extended horizontally. When such gases are deve- loped, as often happens, at night, their expulsion is facilitated by merely sitting upright in the bed, or still more effectually by rising from bed and walking about ; the motion aiding their displacement. The disagreeable odour which sometimes attends these gases proceeds from the vapours of digestion slightly acidulated, with which they are mixed. 657. The Mechanical Action of the Intestines is not less important to digestion than that of the stomach. So long as the food in the stomach is imperfectly digested, the pylorus remains closed so as to prevent its passage into the intestines ; but so soon as the stomachal digestion is completed, this exit is opened, and the food is driven through it into the intestine by the contractile force of the muscles of the stomach. The pylorus is closed and opened, not by a valve, but by a strong muscular apparatus called spivincter, which acts under the stimulus of the food and independently of the will. In the duodenum the food is mixed with the bile and pancreatic juice, as will be hereafter more fully explained. It 444 ANIMAL PHYSICS. passes thence successively into the jejunum and ileum. It- pi ogress through the small intestine is determined by the peristaltic action of that organ, produced by two systems of muscular fibres in its coat, one longitudinal and the other circular. The gases, always developed in a greater or less quantity in the intestines by the process of digestion, facilitate this motion, as air does in the air-vessels of a force-pump. The movements of the intestinal tube are called peristal 1 1*: or vermicular, and consist in the alternate contraction and dila- tation of successive portions of the muscular coat in a wave-like manner. These movements are involuntary, and are excited by the presence of the food in process of digestion. The impression produced by the food, being unperceived and uninfluenced by the will, belongs to the class called by physiologists reflex Fig. 3G9. nervous actions. Nearly all the intestinal and stomachal mo- tions are under the influence of the great sympathetic nerve. A mechanical, chemical, or galvanic excitation applied to the semilunar ganglions (fig. 270,4S), the solar plexus (fig. 210/°), or the splanchnic nerves (fig. 270, 47, 270,45), produces the con- tractions proper to the small intestine. 'When the communica- tion of the great sympathetic with the cerebro-spinal system is cut off, the intestinal functions are suspended. This explains INTESTINES. 445 the sluggish action of the intestines and their occasional paralysis, in maladies of the spinal marrow or the ence- phalon. 658. The Mechanical Action of the large Intestine is analogous to that of the smaller. All that part of the food which has not been absorbed before its arrival at the extremity of the small intestine, passes from the ileum through a valve called the ileo-ccecal valve into the first portion of the large intestine, called the ccecum. The ileum enters the caecum laterally, as shown in fig. 367, a little above the extremity of the caecum, which derives its name from being a cul cle sac, or blind gxd. The part of the great intestine thus called is shown at fig. 367, 9, and lies below the entrance of the ileum into the large intestine. The junction of the ileum i with the large intestine is represented in fig. 369, the caecum and colon being laid open to display the ileo-caecal valve, of which a is the lower, and e the upper fold, c being the caecum, and o the ascending colon. This figure is given by Quain, slightly modified from Santorini. The structure of the valve is such that it opens towards the caecum, the two leaves meeting each other when closed, so as completely to prevent the reflux of the matter into the ileum. The residual matter driven successively through the ascending, trans- verse, and descending division of the colon (fig. 367, u, 12, 13), arrives at the sigmoid flexure. This matter arriving constantly, and in a small quan- tity, at the extremity of the alimentary canal, from which it is expelled only at intervals more or less distant, must be provided with a reservoir, where it can be collected between the epochs of its expulsion. The reservoir is an enlargement of the rectum, forming a sort of ampulla, which lies imme- diately above its termination, as shown in fig. 367. The movements of the large intestine are rendered visible in the living animal by opening the abdomen. They are less pronounced than those of the small intestine, but have the same character. It is in the ascending colon (fig. 347, u) that they are most remarkable. Like the other peristaltic motion, they are under the influence of the great sympathetic. The first portions of the ascending colon are influenced by the solar plexus, fig. 270 5°. The lower part of the rectum is influenced by the hypogastric plexus (fig. 270, 01), which includes filaments from both the great sympa- thetic and cerebro-spinal systems. The movements of the large intestine, in all parts which receive only the filaments of the great sympathetic, are involuntary ; but the lower part of the rectum has a certain sensibility connected with the want of defecation, and its closing muscle, which receives filaments of the cerebro-spinal system, has a certain sensibility, and is under the dominion of the will. 659. The Chemical Phenomena of Digestion have for their ultimate purpose the absorption of the nutritious parts of the food by the organism. The first effect is, therefore, the solution of the alimentary substances ; and when the aliments are not immediately soluble, they are rendered so by the action 446 ANIMAL PHYSICS. of tlie digestive juices. When they are soluble, on the other hand, the action of these juices i3 limited to their simple solution. In this process drinks are powerful aids to the digestive juices. Water itself acts as a solvent on a great number of substances. Alcoholic, fermented, acidulous, and alkaline drinks, severally contribute to the solution of alimentari- matters. These also often act chemically in a manner similar to the digestive juices themselves. The juices secreted in different parts of the alimentary canal have different effects upon the food, but the production of these effects are not always confined to the place where the juices are secreted. The aliment charged with the juices secreted at one point of the canal, progresses onward to other points before the chemical action due to the juices with which it is impregnated takes effect. The consequence of this is, much complication in the chemical effects of digestion. Secondary effects also intervene to increase this complication. Thus, a portion of aliment charged with a certain digestive juice, will react upon other portions not so charged, and produce complicated effects. The digestive juices are five : 1, saliva ; 2, the gastric juice ; 3, the pancreatic juice ; 4, the bile ; and 5, the intes- tinal juice. 660. Insalivation. — While the food introduced into the mouth is broken, triturated, and ground by the jaws and teeth between which it is continually thrown by the mechanical action of the tongue and cheeks, moved by suitable muscles, a liquid called saliva is poured into the mouth from surrounding glands, which moistens the food and reduces it to a pulp. This process constitutes an impox-tant part of digestion ; for although the saliva, as will presently appear, consists of more than 99 per cent, of pure water, the constituents, small as they are in quantity, which it holds in solution, exercise a considerable influence upon the preparation of the aliment for absorption and assimilation. Thus, insalivation not only facilitates the deglutition of the food by reducing it to a moist and soft pulp, which is easily moved through the cesophagus, bxxt prepares it for the chemical changes which it must undergo in the stomach. 661. The Mastication of the food is not only necessary for its due impregnation with saliva, but also to its exposure in INSALIVATION. 447 the alimentary canal to the chemical action of that and other juices, the effect of which in a given time upon a given weight of food will he proportional to the extent of surface in contact with them. If the food be swallowed after imper- fect mastication, and therefore in lumps of greater or less magnitude, the juices, acting only on the surface of each lump without penetrating to the inside of it, will necessarily produce imperfect effects, and the inconvenience and injury attending indigestion must ensue. These circumstances, being well understood, will impress on everyone the importance of perfect mastication, as a condition indispensable to the maintenance of health. 662. Secretion of Saliva. — Among the many admirable and beneficent provisions of nature, there is one attending the secretion of saliva which ought not to be passed without a special notice. Everyone must have felt that the mere recep- tion of food in the mouth, and its contact with the tongue and palate, excites the secretion of this juice, which is always sup- plied in quantities sufficient to reduce the food to a digestible pulp, provided that the operation of mastication be continued to the necessary degree. But even though through undue greediness, haste, neglect, or ignorance, the food be sent into the stomach after imperfect mastication, nature has so ordered it, that the salivary glands continue to secrete their proper juice, which, being swallowed, mixes in the stomach with the food, which ought to have been saturated with it in the process of mastica- tion in the mouth, so that this stomachic insalivation is a pro- vision of nature, if not to prevent, at least in some degree to mitigate, the injurious effects of too greedy or too hasty eating. It further appears by some curious physiological experiments, that food deposited in the stomach, without passing through the mouth at all, as it may be by an artificial opening, actually reacts upon the salivary glands, causing the secretion of saliva, which being swallowed and taken into the stomach, supplies the food with that juice, which it ought to and would have acquired in the process of mastication. 663. Admirable Uses of the Salivary Glands It has been well observed that the salivary glands, not content with dis- charging their functions, when directly excited by food in the mouth, are constantly, so to speak, on the watch to be useful, 448 ANIMAL PHYSICS. and often begin to act the moment the expectation or desire of particular food is entertained. Everyone has felt how the mouth waters at the mere mention of certain agreeable aliments. * 664. Their Number and Position. — In the case of man, there are three pairs of glands placed symmetrically at either side of the median plane ; these are, first, the parotid gland*, situated behind the lower jaw and in front of the ear, the submaxillary glands, which are lodged immediately under the angle of the lower jaw, and, in fine, the sublingual glands , already mentioned, placed immediately under the tongue in the fleshy part of the jaw. Each of these glands communicates with the mouth by a special conduit, pouring the saliva into it. The saliva furnished by each pair of these glands, has a different quality ; that secreted by the parotids being more, and that by the submaxillary less watery, than the saliva of the sublingual glands. Besides these, numerous other less voluminous glands, all of racemose or clustered structure, surround the mouth, such as the glands of the cheeks, lips, in- ferior surface of the tongue, and the velum palati, independently of the numerous follicles appro- priated to the secretion of mucus. The relative position of the submaxillary and sublingual sali- vary glands is shown in fig. 366. The structure of the great sali- vary glands may be illustrated by figs. 370 and 371, which represent the development of the parotid gland of a sheep (reproduced from Midler). 665. The Quantity of Saliva secreted during meals is much greater than in the intervals between them, and is generally in direct proportion to the hardness and dryness of the food. It is estimated that the average quantity secreted by an adult in twenty-four hours is from fifteen to twenty ounces. FIRST APPEARANCE OF THE PAROTID GLAND IN A SHEEP. * Jolmstou's Chemistry of Common Life, vol. i., p. 3W. SALIVA. 449 666. Saliva is a viscous, inodorous liquid, imperfectly transparent, and always alkaline during the reception and mas- Fig. 371. LOBULES OF THE GLAND WITH SALIVARY DUCTS IN A MORE ADVANCED STAGE. tication of food. Its analysis by Berzelius, and more recently by Frerichs, is as follows : In 10,000 Parts of Saliva. Berzelius. Frerichs. Water ....... 9929 9941 Organic matter (ptyaline or salivary diastase) 29 14 Mucus and epithelium ..... 14 21 Fatty matter . . . . . . . . . 1 Alkaline lactates ...... 9 . . Sulpho-cyanuret of potassium . . . . . . i Divers salts ....... 19 10,000 22 10,000 667. Ptyaline. — Of the several solid substances held in solution by saliva, the most important by far, in its effects upon digestion, is that to which Berzelius gave the name ptyaline (from the Greek word for saliva), as being the essential principle of that secretion. Giving to the peculiar process by which that eminent chemist obtained ptyaline, he failed to discover its peculiar physiological virtue, the high tempera- ture to which it was exposed having the effect of altogether effacing that property. More recently Leuchs prepared it by o a 450 ANIMAL PHYSICS. precipitation in alcohol, by which, no elevation of temperature being necessary, the physiological property was unimpaired. Ptyaline owes its physiological importance to its effect on the starch of vegetable food. It produces upon that substance two successive changes. By the first it converts it into a gum- like substance, called dextrine, from an optical property by which it produces right-handed polarisation on light trans- mitted through it. (See “ Handbook of Natural Philosophy,” Optics, § 294.) By the next, it converts this dextrine into another substance, called glucose, or grape-sugar. Dextrine differs from starch only in its state of aggregation, but glucose differs in its constituents, including one equivalent of oxygen and one of hydrogen not in the starch or dextrine. In consequence of this property, discovered by Leuchs, and rendered still more manifest by the experiments of Mialhe, the name salivary diastase has been substituted for ptyaline ; diastase being that constituent of malt in virtue of which the formation of grape-sugar or glucose is accelerated during the fermentation of the worts. 668. Starch is one of the most universally prevalent consti- tuents of vegetable aliment. It occurs abundantly in the grain and seeds of all cereals, and in many roots, such as the potato and arrow-root. It is also found, according to Liebig, in unripe apples and pears, and in the seeds of leguminous plants. It is most easily obtained pure from potatoes. Starch itself is insoluble, and would therefore be indigestible ; hence the importance of the chemical change produced upon it by the saliva, by which it is converted into grape-sugar, a substance eminently soluble. It belongs to the non- nitrogenised class of aliments, and therefore supplies no elements for the increase of the body or its organs, being useful chiefly in the production of animal heat. It composes a part of the fuel of the organism. Whatever part of it is not con- sumed in organic combustion accumulates in the form of fat. 669. The Salivary Glands do not all secrete saliva of the same quality. M. Lassaigne ascertained that the saliva of the parotid gland of the horse has not the power of converting starch into sugar. M. Bernard showed that the same is true of the saliva secreted separately and conjointly by the parotid and sublingual glands of the dog. It appears from these and other results that the virtue by which the saliva renders vege- table food (including bread) digestible, is a property of the ACTION OF SALIYA. 451 products of all the glands of the mouth taken collectively, and that the peculiar ferment of ptyaline proceeds from the smaller rather than from the chief glands. As the conversion of starch into sugar by ptyaline is not instantaneous, but on the contrary requires a considerable interval, and as the sojourn of food in the mouth is much more brief, it follows that the saccharine transformation must take place chiefly in the stomach, a fact which raised some doubts as to the phenomenon. The alkalinity of the saliva is a condition indispensable to the saccharine change. Now the moment the pulp enters the stomach it is brought under the action of the gastric juice, which is acid. The first effect of this must be the neutralisation of the alkaline pulp, and the next its acquisition of the acid principle. It was therefore contended that such a change would at once arrest its saccharification. This reasoning, however, has been proved erroneous : first, by the experiments of Schwann, which were repeated and varied by Jacobowitsch, Frerichs, and others, and which can be easily reproduced. The acidification of the pulp only retards, but does not arrest its saccharification ; and as it remains for some hours in the stomach, abundant time is given for the completion of the latter process. In relation to these effects it may be observed, that in the case of ruminants, which derive their nourishment chiefly from fecu- lent aliments, the food is largely supplied with saliva, being re- peatedly masticated in being transferred from stomach to stomach. 670. That neither the fatty parts of the food, such as fat, oil, or butter, nor the nitrogenous aliments, are affected by the saliva, Ls proved by experiments on artificial digestion. The part played, therefore, by that fluid is strictly limited to the specific action of the ptyaline on the feculent constituents, and the general dissolving action of the large proportion of water of which it consists. With carnivora, therefore, which consume in general no feculent food, it acts merely as a mechanical agent in converting the food into pulp, and thus facilitating deglutition. Cases have occurred in disease in which, not only insalivation, but all mastication has been suspended for months. In the Maiaon dc SantS of Dr. Blanche, at Passy, near Paris, an insane patient obstinately refused to swallow food of any kind, and was for many months nourished altogether by food passed artificially into the stomach, through the oesophagus, without deglutition. This patient did not even swallow his saliva. It was necessary to remove from his mouth, two or three times a day, the accu- mulation of saliva by which it was distended. Food consisting of a due proportion of nitrogenised, fatty, saccharine, and feculent aliments, united Q a 2 452 ANIMAL PHYSICS. with a certain quantity of vegetable diastase, to' supply the place of saliva, was passed into the stomach by means of a tube. Nevertheless the physical health of this individual was perfect, and even his weight augmented under this regimen. This singular phenomenon may perhaps be explained by the fact which will presently appear, that the feculent aliments which resist stomachal digestion encounter agents lower down in the canal which act upon them. 671. Stomachal Digestion. — The stomach, as already ex- plained, is a membranous bag, placed across the upper part of the cavity of the ab- domen, and almost im- mediately below the diaphragm. It has c the form of a bagpipe, and it is with the stomach of animals having this shape that the air reservoir of the bagpipe is made. When distended, its shape Fig. 372. is that which is represen- ted in outline in fig. 372. The left side, c, is called the great cul de sac, or fundus, the right, d, the small cul de sac, or antrum pylori. The lower part, a, is called the great curvature, and the upper, b, the lesser curvature. The opening o is united to the lower end of the oesophagus, at or near the point where the latter passes through the diaphragm. This, which is the entrance to the stomach, is called the cardia, or cardiac orifice. The opening, q, on the right, leads to the duodenum, and, as already explained, is called the pylorus. The stomach is considered as consisting of three regions, limited by the dotted lines in the figure. The left, c, is called the great or splenic end ; the right, d, or smaller end, is called the pyloric region. 672. Gastric Juice. — The stomach, like all other parts of the intestines, is lined by the mucous membrane, which, as else- where, secretes the liquid called mucus, by which its surface is everywhere and at all times lubricated. When the stomach is empty, and the process of digestion suspended, this mucus is the only liquid secreted. But when any substance is intro- duced into the stomach, whether it be food or not, it excites a peculiar secretion called the gastric juice, which plays an important part in stomachal digestion. The gastric juice is a colourless, limpid liquid, having a faint odour characteristic of the annual which secretes it, and a slightly saltish flavour. Its specific gravity differs but little from that of water, being heavier than it in the human STOMACHAL DIGESTION. 453 organism by no more than a 20th per cent. Tested by turmeric paper it betrays the acid quality. Submitted to analysis it is found to contain 99 per cent, of water, combined in a minute proportion with certain salts, a free acid, called lactic acid, and a peculiar organic substance which has received the name of pepsine. Tlie acidity of the gastric juice was, until a late period, ascribed to the presence of hydrochloric acid. This, however, was shown to be an error arising from the process by which it was prepared. M. Chevreuil first showed the true character of the acid, which was still more satisfactorily demonstrated by M. Lehmann. The first physiological experiments on the gastric juice were made by forcing dry sponges into the stomach of live animals through the oesophagus. These becoming saturated with the gastric juice, were either withdrawn by means of cords, or removed by putting the animal to death. The pro- cesses, however, practised more recently by M. Blondlot are much more effectual, inasmuch as they enable tbe observer to examine the series of phenomena arising from the development of the gastric juice in all stages of digestion. This process consists in the production of an artificial gastric fistula, by which a communication is made from without with the interior of the stomach. This is accomplished by making an incision in the epi- gastric region, drawing out the stomach, opening it by an incision, and fixing the edges of such incision to the lips of the abdominal incision by suture. At the end of some days, adhesive inflammation has ensued, and the opening of the stomach is permanently connected with the abdominal open- ing, so that the communication from without is established. A canula is inserted in the opening, which is stopped by a cork. By the aid of this ingenious contrivance, the gastric juice can be produced at will, and aliments can be introduced into and withdrawn from the stomach at all stages of digestion, so as to enable the physiologist to study the series of transformations which they undergo. In some rare and exceptional cases, pathological lesions have produced fistula; of the same kind in the human patient, so that similar observations have been made in the human stomach in the process of digestion. 67 3. A multitude of glands, simple and compound, of differ- ent structures, are diffused everywhere through the coats of the stomach. It is, however, chiefly in the pyloric region that the compound glands are found ; those round the cardiac opening and along the lesser curvature being chiefly an agglomeration of mucous follicles. It is probable that these different glands secrete different qualities of juice, as is ascertained to be the case in the salivary glands ; but this fact has not been esta- blished by experiment, the gastric juice thus obtained being always the result of the general secretion of the organ. From analogies derived from observation on ruminating animals, it seems probable, however, that the most efficacious part of the gastric juice is developed in the pyloric region, since it is found 454 ANIMAL PHYSICS. that in ruminants the gastric juice, properly so called, is secreted exclusively in the last stomach, which corresponds to the pyloric side of the human stomach. 674. The Constituents of the Gastric Juice which exercise the most important influence on digestion are the lactic aci/l and the pepsine. Other acids, such as acetic and butyric, are, it is true, sometimes found in the stomach, but these are not properly secreted, but result from the phenomena of digestion, and are never found except when this function is in progress. To obtain the gastric juice in its normal condition, unmixed with foreign or accidental constituents, it should therefore be taken from the stomach of an animal in which the process of digestion is suspended. 675. Pepsine, called also by some physiologists chymosinc, and by others gasterase, is a nitrogenised substance having some analogy with albumen, and having the character of a ferment. It is soluble in water, and insoluble in alcohol, which precipi- tates it. It is also precipitated by tannin, and by the acetate of lead. It differs from albumen in this, that its aqueous solution is not rendered turbid by ebullition ; nevertheless, though not coagulated, an elevated temperature deprives it of its characteristic properties, and hence have arisen some errors of physiologists, who have experimented on the gastric juice at high temperatures. The best process for producing pure pepsine is that of M. Payen, who proceeds thus : — He procures the gastric juice of a dog, taken from the stomach, as usual, by an artificial fistula. The pepsine is precipitated by alcohol, and ■with it some small proportion of albumen and mucus. This precipitate is treated with water, which dissolves only the pepsine. The solution of pepsine is again precipitated by alcohol, and the precipi- tate dried at a temperature of 100°. The following is the analysis of the gastric juices of the horse and dog : — Horse. TicdemanniGmelin. Dog. Frerichs. Water ...... Organic matter . . . . • Salts ...... 9S-10 105 0-55 9S-S5 0 72 043 The phenomena developed in stomachic digestion by the action of the gastric juice upon the food, are ascertained partly by artificial digestion, ARTIFICIAL DIGESTION. 455 and partly by observing the state of the food taken directly from the animal stomach in different stages of digestion. 676. Artificial Digestion is produced by merely exposing the different kinds of aliment to the gastric juice, natural or artificial, at the temperature of the living body. In all such experiments it is remarked that the aliments, when divided into very small fragments, are much more quickly dissolved by the gastric juice than when exposed to it in larger pieces. The utility of mastication, and of the mechanical action of the stomach is thus rendered evident. Artificial gastric juice, for experimental purposes, may be produced by dissolving pepsine in acidulated water. 677. Aliments consisting of fibrine, gluten, or coagulated albumen, submitted to artificial digestion, are dissolved after the lapse of some hours, the product of the solution being the same in each case. When the same experiment is made with caseine, coagula- tion first ensues, produced by the acidity of the juice. To this succeeds, by degrees, disintegration and finally complete solution. The ultimate product is no longer coagulable either by acids or by heat. Liquid albumen is rendered slightly turbid by the gastric juice, though it cannot be said to be coagulated. This turbid appearance, however, soon ceases, and at the end of five or six hours it undergoes, like other albuminous substances, an isomeric * transformation. It is no longer coagulable either by acids or by heat. When gelatine, obtained in the jelly of meat or bones, is exposed to the action of the gastric juice, it is soon dissolved, forming a clear brown liquid. The product, however, is not a pure and simple solution, since, when concentrated by evapora- tion, it is found to have lost the property which it possessed of resuming the state of jelly when cold. It is not certainly known whether the production of the solution of gelatine by the gastric juice is identical with the results of the like solu- tions of the other aliments above mentioned. It follows, therefore, in general, that fibrine, gluten, albu- men in the solid or liquid state, and caseine, are dissolved and metamorphosed by the gastric juice into an identical substance. This final result of the process has the same chemical * Substances are said to undergo isomeric changes when their properties are altered, while their chemical constituents remain the same. 456 ANIMAL PHYSICS. composition as the albuminous substances from whence it proceeds, as has been proved by the analyses of M. Lehmann. Like albu- minous substances in general, this forms xanthoproteic acid when heated with nitric acid. It is precipitated by alcohol, tannin, and corrosive sublimate. It differs from albumen properly so called, inasmuch as it is not precipitated by acids or coagulated by heat. This common product of the stomachic digestion of albu- minous substances is called peptone by Lehmann, and alburn inose by Mialhe. 678. The alimentary matters which are not albuminous are not attacked or dissolved by the gastric juice. Thus fatty substances and oil remain altogether unaltered by it. In the artificial digestion of meat, the fat is seen floating at the surface in an oily stratum. Neither starch nor sugar is affected by the gastric juice in any manner different from that in which they are affected by most of the juices of the economy. Cane-sugar is transformed into grape-sugar, and in that state is absorbed ; but this change does not take place in the stomach, being produced subsequently in the small intestine. Neither gum nor pectine are affected by the gastric juice. All organic substances soluble in water, such as the chlorides and the alkaline phosphates and sulphates, are also soluble in the gastric juice, their solution being facilitated by the aqueous drinks which they gene- rally encounter in the stomach. The phosphate of magnesia and such salts of lime, iron, and other similar principles, which are scarcely soluble in water, are rendered so by the acidity of the gastric juice. In comparing artificial with natural digestion, it is important to remark, that though their ultimate results are identical, the natural digestion is always more prompt than the artificial. Numerous experiments made by M. Blondlot demonstrate this. 679. Natural Stomachal Digestion. — If the stomach of the animal be opened at different periods of the process of digestion, and its contents examined, they will be foimd to consist of a sort of paste, pulp, or brothy substance called ch ytne, composed of the food combined with the gastric and salivary juices, and which will be more or less complex according to the food which the animal has taken, and more or less liquefied according to the quantity of drink which it may have swallowed, and accord- ing to the progress, more or less advanced, of the digestive process. If we suppose, for example, that the animal has taken a mixed food, consisting of milk, bread, meat, potatoes, and vegetables, we shall find, in the first place, the contents of the STOMACHAL DIGESTION. 457 stomach to consist of starch not yet transformed, and which will not he so until they pass into the intestine, with dextrine and sugar proceeding from the partial action of the saliva upon a certain quantity of starch. The salivary action, which com- mences in the mouth, is continued in the stomach, but is not completed until after the food has passed through the pylorus into the intestine. Much of the food will also be found in the stomach, not yet modified by either the saliva or the gastric juice, consisting of fat, which resists both of these juices ; albuminous substances, such as fibrine or caseine in different degrees of solution ; but if the examination take place two or three hours after a meal, such substances will have in a great degree disappeared, being absorbed by the coats of the stomach. A considerable quantity of the constituents of food will also be found, which are not susceptible of con- version either by the saliva or by the gastric juice, or even by any of the juices secreted in the lower parts of the alimentary canal, such as cellular substance, vegetable fibre, grains of starch which have not been broken, fragments of tendons, and so on. Gastric juice, not yet combined with the food, and lactic acid, will also be found. A considerable quantity of the latter acid will also be accumulated, proceeding from the decomposition of milk-sugar, and other saccharine principles of the food. This acid also sometimes proceeds from the starch of the food, transformed into grape-sugar in the stomach. Acetic acid, proceeding from the peculiar fermentation of sugar, is also sometimes found in the products of stomachal digestion. The same acid also proceeds from wine and alco- holic liquors, especially when taken in excess. 680. In comparing the effects of artificial digestion produced in phials, with the natural digestion of the stomach, erroneous inferences may be drawn if the peculiar absorbing power of the coats of the stomach be not taken into account. In the artificial process, the products resulting from the gradual combination of the gastric juice with the food still remain in the phial, and may probably, by their continued presence, produce important modifications in the results. But in the stomach the food combined with the gastric juice being in immediate contact with an absorbing surface, passes, by absorption, into the vascular system, and is thus removed from the stomach, so as to prevent the occurrence of those secondary effects which would be mani- fested in artificial digestion. 458 ANIMAL PHYSICS. An example of the importance of this distinction was presented in the experimentary researches of M. Blondlot, who, having introduced into the stomach of an animal through a gastric fistula liquid albumen, found that it disappeared almost immediately by absorption, whereas the same liquid albumen, put into contact with the gastric juice in the phials used in arti- ficial digestion, was transformed at the end of five or six hours into the substance incoagulable by heat called peptone. It is, therefore, extremely probable that the liquid albumen taken into the stomach is always pre- vented from undergoing the metamorphosis by prompt absorption ; and that, consequently, the only substances transformed into peptone, or albu- minose, in the living stomach, are solid albuminous matter, such as fibrine, coagulated albumen, whether animal or vegetable, leguminous gluten, and so on. It must be added, however, that albumen in the liquid state is scarcely ever taken into the human stomach — the animal substances on which man feeds being submitted generally to a culinary process which removes that principle. 681. Digestibility of rood. — In determining the relative digestibility of different species of food, a question often pro- posed to the medical practitioner, it is important to take into consideration the fact that the process of digestion is not con- fined to the stomach, but is carried on with more or less energy throughout the whole extent of the intestinal canal. It would, therefore, be a great error to assume that food which passes easily and promptly through the stomach, is, therefore, more digestible than other forms of food which are retained there for a more considerable time. Nature has so ordered it that food which is not attacked by the gastric juice, but which is suscep- tible, either exclusively or principally, of intestinal digestion, soon passes from the stomach to those parts of the alimentary canal where it is destined to meet with juices capable of con- verting it. Aliment, therefore, which is most digestible is that which most promptly yields to the action of the digestive juices, in whatever part of the canal it may be affected by them. Among the aliments which pass most promptly through the stomach, ax-e those of vegetable food. If an animal be supplied at the same meal with meat and vegetables, the stomach will retain the former, while the latter passes on. Drinks of every sort, which have no need of combination with the gastric juice, and which are absorbed indifferently in all parts of the alimen- tary canal, pass veiy promptly through the stomach. 682. It may be generally stated that vegetable food is less digestible than animal, as indeed must be expected from con- sidering that the latter is in a state more assimilated to the organs of the body than the former. In vegetable food are DIGESTIBILITY OF FOOD. 459 found the most refractory constituents relatively to the digestive functions, such as cellular fibre, the envelopes of grapes, lentils, peas, beans, apples, pears, and fruit generally. Most vegetables which have not been either mashed in the culinary process, or strongly masticated in the mouth, are found to be presented in their natural state in defecation. Among the most indigestible food taken into the stomach are truffles and mushrooms. 683. There are some sorts of aliments which not only are incapable of stomachal digestion, but, by their continuance in the stomach, obstruct the digestion of other forms of food, especially when taken in any considerable quantity. Such are animal fat generally, butter, oil, the oily principle of walnuts, almonds, hazel-nuts, and olives. These and the like, even when they pass from the stomach to the intestine, are extremely difficult of digestion, and are incapable of any but the slowest absorption. When taken in any considerable quantity, they pass through the alimentary canal altogether undigested. 684. So far as relates to the relative digestibility of the different constituents of food, determined by the mean time required for chylification, the following are the results of the experiments made by M. Blondlot upon dogs by means of the gastric fistula. Aliment. Time op Digestion. Fibrine ....... Gluten (cooked or baked) . . . . . Caseine ....... Coagulated albumen . . . . . Fibrous tissues (tendons, ligaments, Sic.) Hours. Minutes. 1 30 2 0 3 30 6 0 10 0 685. Experiments of Dr. Beaumont. — Among the phy- siological experiments on digestion, those of Dr. Beaumont, an American physician, merit especial notice. These were made upon a young man whose stomach had been opened by a gunshot wound, which, though cicatrised, still remained sufficiently open to render the stomach distinctly visible. Dr. Beaumont ascertained that, immediately on the arrival of food in the stomach, the secretion of the gastric juice commenced, and, by its intermixture with that fluid, after a sufficient interval, the food was converted into chyme ; ho not only took out of the stomach of this man the mixture of food and gastric juice and saw it converted into chyme separate from 460 ANIMAL PHYSICS. Lis body, but be was enabled, by means of a tube, to procure a certain quantity of tbe gastric juice, which he saw oozing through the sides of the stomach, and with this juice, like Spallanzani, he succeeded in exhibiting artificial digestion. He mixed it with pieces of beef, properly triturated, and exposing the mixture to the proper temperature, saw it converted into chyme. The following are among the results obtained by observations on this patient : — Aliments. Time of Digestion. Veal, beef, mutton, and pork (whether boiled or fried) Ditto (roasted) . Fowl — brown (goose, duck, &e.) . Fowl — white (chicken, pheasant, &c ) Fish . Hours. Minutes. 4 0 3 30 3 30 3 0 2 30 C86. Much uncertainty attends the relative digestibility of feculent aliments, such as bread, pastry, potatoes, ( Gelatine ’ | Mineral matter . Water * • lbs. . IS 6 • 4| • H . 116 154 Thus it appears that about 7 5 -7 per cent., or a little more than three-fourths of the entire weight of the human body, consists of water, the remainder being dry solid matter, with which the water is combined, and part of which it renders fluid. The total weight of the liquid blood is about 20 lbs. in the case of such an average man as is here sup- posed, and of this 15-5; lbs. is water. That substance, there- fore, enters the blood in the same proportion as that in which it enters the entire body. As the whole nutriment of the * Johnston’s Chemistry of Common Life, voL ii. p. 3t5. CONSTITUENTS OF THE BODY. 471 blood is necessarily contained in the substances held in solution by this 15jj lbs. of water, it follows that the entire stock of nutriment for the maintenance of the body, contained at any given moment in the blood, amounts to no more than 4-y lbs. Exclusive of the water with which the remainder of the body is impregnated, the entire weight of organised matter contained in it, including flesh, skin, and bones, amounts to only 33f lbs., and the stock of nutriment by which this is maintained, is only the 41.- lbs. of solid organic matter contained in the blood. This nutritious part of the blood, therefore, is required to maintain and constantly to replace the waste of about eight times its own weight of organised matter in the body. It is evident from this, that the total quantity of organic matter in the blood must be renewed eight times as fast as that which composes the rest of the body. 710. Mammifers. — In this division of the animal kingdom, the functions of nutrition and the structure of the alimentary canal are, in their general character, similar to the corresponding fimctions and parts of the human organism. In particular cases there are, nevertheless, peculiar differences, which merit attention. 711. Mastication is an operation common to nearly all mammifers ; but the instruments by which it is performed vary with the food upon which they are intended to operate. So complete is this harmony between the organs and the fimctions, that by the mere inspection of the masticating apparatus, or even by a single tooth, the regimen, habits, and the general structure of the animal to whom it has belonged can be deter- mined. Thus, by the tooth represented in fig. 375, we can infer with certainty that the animal to which it belonged must have had a bony skeleton fitted to carry such a tooth, and also to support all the parts of its body. Now, such a skeleton could not exist without having a cerebro-spinal axis to protect it. The animal must, therefore, have had a brain and its appendages, a spinal cord and a nervous system ; and this brain and nervous system necessarily suggest the existence of organs of sense, establishing a communica- tion between the animal and the external world. The structure of the tooth, moreover, indicates that the animal was provided with a complete circulatory ap- paratus, and bones sufficiently developed to afford a safe lodgment for such a tooth — a character which is only found in certain quadrupeds. It would therefore ” “ ' be inferred that the animal in question was a quadruped and a mammifer. Fig. 375. TOOTH OB’ A I.inv 472 ANIMAL PHYSICS. From the form of the tooth it would also be inferred tliat it belonged to a carnivorous quadruped, for no graminivorous nor even omnivorous animal has teeth of this form. The food of the animal was, therefore, flesh, and consequently the stomach and intestines must have been formed in such a manner as to digest this aliment ; and the animal must have been fur- nished with organs of locomotion and prehension proper for the seizure of its prey. Thus pursuing the chain of reasoning from consequence to consequence, based upon the known analogies of nature, we arrive, if not at an exact knowledge of the structure of the individual animal, at least at the dis- covery of its most salient characters and functions. * 712. Nothing can be more striking than the difference between the mas- ticating apparatus of animals whose nourishment consists of different substances. In those which live on flesh, the molar teeth are compressed and sharp, and so placed as to meet each other like the blades of a scissors ; while the canines are largely developed, and the incisors resemble them more or less in form (fig. 376). 713. In those animals which feed on insects the teeth are formed into a series of conical points ; those of one jaw fitting between those of the other, like the teeth of wheelwork (fig. 377). ' TEETH OF A CARNIVOROUS ANIMAL. Fig. 377. TEETH OF AN INSECTIVOROUS ANIMAL. Fig. 37S. TEETH OF A FRUGIVOROUS ANIMAL. 714. When the animal feeds principally on soft fruits, the teeth are principally formed with rounded tubercles, as shown in fig. 378. When animals feed on vegetable substances more or less hard, such as grain, the teeth are terminated by large flat surfaces (fig. 379) ; those of one jaw clos- ing upon those of the other, so as to bruise the food between them ; by a lateral motion of the jaws one upon the other, imparted bv the Fig. 3,9. -TEETH OF ORAMI- maxillary muscles, the food is submitted to nivorous animals. trituration, like that which grain suffers between two millstones. It has been from this peculiar action that certain teeth have received the name of molars. Edwards’s Zoology, p. 273. INFERIOR ANIMALS. 473 715. Of all forms of teeth, the molars are the most universally useful, and they are accordingly more prevalent than either incisors or canines. These last, heing necessary to seize and devour a living prey, are never wanting in carnivorous species. But they are much less useful, and there- fore altogether wanting, in herbivorous species. Thus, in the horse, the canine teeth are merely rudimentary, and the incisors have the structure of molars. Sometimes the canines, losing their character of masticators, become a weapon of offence or defence ; as, for example, in the case of the boar (fig. 380). _ Fig. 380. The three classes of teeth — incisor, jaws of the boar. canine, and molar — are found com- bined in asses, carnivora, ruminants, and most of the pachydermata. In ruminants and pachydermata there is, besides, an interruption in the series of teeth formed by what are called bars ; and this class has no incisors in the upper jaw. The ruminants without horns have no canines. The rodents have only molars and elongated canines, without incisors. The molars, which are the true instruments of mastication, are the teeth which are most rarely absent in animals. The rhinoceros and elephant have no other form of teeth. There are also mammifers destitute altogether of teeth, of which the ant-eaters (Myrmecophaga), pangolins, echidnas — some- times called spiny ant-eaters — and whales, are examples. The two sides of the npper jaw of the whale, which is keel-shaped, are furnished with two thin transverse serrated lamellae, called balene or whalebone, con- sisting of a sort of fibrous horn, fringed at the edges, which serve to retain and strain from the water the minute animals on which these enor- mous cetaceans feed. 716. Nutrition of the Lower Animals — Prehension of Food, — Vegetables receive their nourishment immediately from Fig. 381. suitable material substances which surround them. Animals, 474 ANIMAL PHYSICS. however, are generally obliged to seek their fowl, and some- times avail themselves of the use of their prehensile members to lay hold of it and to convey it into their digestive apparatus and also use their locomotive members to pursue it. Thus man uses his hands to convey fowl to the mouth, and many of the inferior animals, such as monkeys, use their prehensile members in the same manner. Certain animals, whose bodily motions are slow, and the opening of the mouth very limited, seize their prey by means of the tongue, which is long and capable of being darted out to a considerable distance ; others, such as the elephant, are provided with a prolongation of the nose or snout, which being very flexible, and having the power of suction, attaches itself to the alimentary objects, which it lifts into its mouth. Others, again, are provided with long and thin processes surrounding the mouth, by which the food is seized, called “palpi,” or feelers in the case of insects, (figs. 383, 384), and “ tentacula ” in the case of mollusca (fig. 385) and polyps (fig. 386). Fig. 383. carabus (Beetle). Fig. 3S4. FALI'I OF CARA ECS. 717. Means of drawing in Liquid Nourishment. — Liquid nourishment is either poured into the digestive apparatus, into which it descends by its gravity, or is drawn in by suction, or, in fine, by both these operations combined. Suction is sometimes produced by the same action of the chest as that by which air is drawn into the lungs. The suction, however, is often performed by the motion of the tongue in the mouth, which acts in the same manner as the piston does in a syringe. The lips being applied MECHANISM OP SUCKING. 475 Fig. 385. MOLLUSCA (Calamary). Fig. 3S6. HYDRA, OR FRESH-WATER POLYP. back like the piston in the syringe, the liquid is forced by the atmo- spheric pressure acting from without into the mouth. It is this curious pneumatical experiment which the new-born infant performs when it sucks the mother’s breast. The partial vacuum produced in the mouth by the retractation of the tongue, gives effect to the atmos- pheric pressure acting on the soft integument of the breast, by which the milk is forced through the orifice in the nipple into the mouth of the infant. The same explana- tion is, of course, applicable to the case of all animals that suckle their young. 718. The Suckers of In- sects.— Some of the inferior ani- 3S~- mals, among which are many species of insects, draw their nourishment either from juices which they find in plants, or from the bodies of other animals upon which they live as parasites. The openi ng of the alimentary canal in these species is gene- rally a tube or sucker, by the aid of which they draw the juices, as they would draw them with a syringe. 476 ANIMAL PHYSICS. 719. Digestion of Animals generally. — In the animal series the digestive apparatus consists of an internal cavity, in which the aliment is submitted to those chemical changes which render it fit to nourish the organism. This cavity is generally tubular in form, and open at both ends, the aliment being taken in at one end, and its undigested residuum dismissed at the other. 720. Mammifers. — The salivary apparatus of this class, like that of Man, consists of parotid, maxillary, and sublingual glands. In most of the ruminants the mucous membrane of the cheeks, including the upper and lower molar glands, are considerably developed. Herbivora, subsisting upon feculent food, which requires much saliva for its digestion, have the salivary glands highly developed. In carnivora the digestive canal is shorter and less complex than in herbivora, an obvious consequence of the more easy digestibility of the food on which they subsist. In carnivora the ctecum is small, while in herbivora it is remarkable for its length and capacity. In the horse it measures thirty, and in the beaver twenty-four, inches. In herbivora the stomach is generally complex in its structure, and large in its capacity, so as to be suited for the reception and digestion of a species of food which within a great volume contains little nutriment. It is in ruminants that this complexity of structure is most remarkable. The transition from the single stomach of carnivora to the quadruple organ of ruminants, is remarkable. In the solidungula (the horse, ass, &c.) the stomach, though simple, is distinguished by the different structure of different parts of it. In the pachyderms it has peculiar appendages, or sack-like dilatations. In several of the rodents, such as the hamsters and water-rats, it consists of tw. parts, in the great kangaroo of three, and in the sloth of four. It is, however, in the ruminants that this organ presents the most curious complexity of form. 721. Ruminants in general have in the alimentary apparatus four cavities or enlargements, which have received the name of stomachs, the food being first received into one or more of them with little or no previous mastication, where it undergoes the first process of digestion. It is then ejected through the oesophagus, and brought back to the mouth, where it is well masticated and triturated ; and, being reduced by impregnation with saliva to a semi-liquid state, it undergoes a second deglutition, and is passed into the other stomachs, where it is brought under the operation of the gastric juice, and then sent through the intestines, as in man. Rumination is the name given to this part of the digestive process, which consists in ejecting the imperfectly masticated food from the first stomach, and, after mastication, transferring it to the succeeding stomachs. 722. The Paunch, sometimes called the rumen — a Latin word of like signification — is the first stomach into which the food is received, and is the largest cavity of the digestive apparatus. Its internal surface is lined with a thick epithelium, and, like the skin, is papillary in its structure. INFERIOR ANIMALS. 477 It occupies a large space on the left side of the abdomen. In this com- partment the food is submitted to a sort of maceration, produced by its admixture with the saliva. 723. The Reticulum, or second stomach, is much smaller, and is in communication with the paunch. It is distinguished by the honeycomb structure and denticulated folds of its lining membrane. 724. The Manyplies, Omasum, or third stomach, has a lining consisting of deep longitudinal folds of the mucous membrane, arranged side by side like the leaves of a book. 725. The Rennet, or Abomasum, as the fourth stomach is called, has an elongated form, as if it were merely an enlarged part of the intes- tine. The mucous membrane which lines it is soft. The food having been softened by maceration in the first two stomachs, is, after a certain in- terval, returned through the oesophagus to the mouth, where it is again masticated ; after which it descends through the oesophagus into the third stomach, and thence by a narrow opening into the fourth. The first and second stomachs may be considered as diverticula of the cardiac portions of the true stomach. The canal by which they communi- cate with the oesophagus, called the (esophageal groove, admits of being closed and formed into a tube, through which the food passes onwards to the third stomach, after rumination, without entering the first or second. The stomachs of a sheep are shown in fig. 388, and also in section in fig. 389. 726. It appears that the imperfectly masticated parts of solid food first swallowed pass into the first and second stomachs. All liquid and semi- liquid food, when swallowed, passes directly to the third, and thence to the fourth or true stomach, which alone secretes the gastric juice. This curious circumstance has been explained upon merely mechanical prin- ciples by M. Flourens. It has resulted from the experimental researches of that eminent physiologist, that these phenomena are effects of the structure of the oesophageal groove. That apparatus consists of a mem- branous tube, leading from the oesophagus to the third stomach. The side of this tube is slit open longitudinally, but the edges of this opening remain united, except when opened by some adequate mechanical force. Liquid or semi-liquid aliment descending through the oesophagus does not exert a pressure of sufficient force to open this slit, and it consequently passes directly from the oesophagus to the third stomach. Hence all liquid food first swallowed by the animal passes directly to the third stomach without entering the first or second. The food ejected in rumination, being masticated and triturated in the mouth, and impregnated with saliva and thus reduced to a semi-liquid state, also passes along the oeso- phageal tube without opening the slit, or passing into the first or second stomachs. It is otherwise, however, with the solid and imperfectly mas- ticated food which is first swallowed by the animal. This, descending from the oesophagus in gross masses, forces open the slit in the oesophageal tube in passing through it, and thus falls into the first and second stomachs. The regurgitation of the food from the first and second stomachs to the mouth, for the purpose of rumination, has been generally ascribed to the contractile action of the second stomach. It appears, however, from the 478 ANIMAL PHYSICS. researches of M. Flourens, that this contractile action is also exercised hr the membrane of the first stomach. The food in these two cavities is hr this mechanical force pressed through the slit-like opening in the ceso* Fig. 3S8. STOMACHS OF A SHEEP. phageal canal, the contractile action of which presses it in pellets or balls towards the oesophagus, which, in its turn exercising a similar contractile force, presses these pellets onwards to the mouth, where the food is, by mastication and intermixture with saliva, brought to a semi-liquid state. It is swallowed, and, as already observed, passes through the oesophageal canal, without opening the slit, into the third stomach.* In cetacea, the complex structure is found, as well in those which feed on animal as in those which subsist on vegetable food. In the latter, the stomach has several compartments ; and in the former it has five, and often more, divisions. During the early part of life herbivora necessarily live on animal food, since they are supported by the milk of their mother. * Other authorities affirm that this exclusive division of labour between the several stomachs does not take place. According to them, a part of the liquid and semi-liquid food first swallowed passes into the first and second stomachs through tlie slit in the groove ; and they further maintain that even the ruminated food, when swallowed the second time, falls partly into the first and second stomachs, from which it passes into the others. (See Boclani. Traitc do Physiologic, p. 121.) Neither the physiological functions nor the mechanical action of tlie stomachal apparatus, nor oven the course of the food in passing through it, are clearly or satisfactorily made out. FOOD IN RELATION TO STRUCTURE. 479 It may then be asked how it happens that the organism — adapted, as we have seen, to the difficult digestion of vegetable food — is also suited during this period of life to that of animal aliment. Nothing can be more Fig. 389. stomachs op a shbep (Section). admirable, however, than the contrivance of nature to adapt the structure of the animal to these varying circumstances. Until a certain age, the first stomach, which in the full-grown animal has great capacity, is small ; and it is not until the animal arrives at that stage of its growth at which it begins to feed on vegetables that the stomach is enlarged so as to be adapted to the new character of its aliment. 727. The changes in the alimentary canal of certain am- phibia, such as the frog, are still more remarkable. The amphibia in the larva state have an internal canal of great length, and during that time they feed principally on vegetable substances. At a later period, when the character of their aliment is changed, the intestine becomes short. 728. The intimate connection thus existing between the organisation of an animal and the nature of its food, which will appear hereafter in a still more conspicuous manner, has been thus beautifully illustrated by Cuvier. “Every organised being forms a single and perfect system, the parts of which mutually correspond, and concur in the same definitive operation by reciprocal reaction. None of these parts can change without the whole changing ; and, consequently, each of them considered sepa- rately indicates all the others. Thus, if the intestines of an animal are organised to digest raw flesh alone, it follows that its jaws must be 480 ANIMAL PHYSICS. constructed to devour a prey, its claws to seize and tear it, its teeth to cut and divide it, the whole structure of the organs of motion such as to pursue and catch it, and its perceptive organs to discern it at a distance. Nature must have placed in its brain the instructive organs to prompt it to conceal itself, and lay snares for its victims. That the jaw may be enabled to seize, it must have a certain peculiar form of articulation, so as to give the leverage to the muscular power which is necessary and sufficient for the resistance ; a certain volume must be given to the temporal muscle, requir- ing an equivalent extent in the cavity which receives it, and a certain con- vexity of the zygomatic arch under which it passes : this zygomatic arch must also possess a certain strength to give force to the masseter muscle. That an animal may carry off liis prey, a certain strength is requisite in the muscles which raise the head ; whence results a determinate forma- tion in the vertebrae to which the muscles are attached, and in the occiput in which they are inserted. That the teeth may cut the flesh, they must be sharp ; and more or less so, according as they will have, more or less exclusively, flesh to cut. Their roots must be the more solid as they have more and larger bones to break. These several circumstances will in like manner influence the development of all those parts which serve to move the jaw. That the claws may seize the prey, they must have a certain mobility in the talons, and a certain strength in the nails, whence will result determinate formations in all the claws, and in the distribution of muscles and tendons. It will, moreover, be necessary that the fore-arm have a certain facility of turning, whence again will result a determinate formation of the bones which compose it ; but the bone of the fore-arm articulating in the shoulder-bone, cannot change its structure without this latter also being modified. In a word, the formation of the tooth bespeaks the structure of the articulation of the jaw ; and that of the scapula, the structure of the claws ; just as the equation of a curve involves all its properties ; and, in taking each property separately as the basis of a par- ticular equation, we should find again both the ordinary equation and all the other peculiar properties. So the claw, the scapula, the articulation of the jaw, the thigh-bone, and all other bones separately considered, require the peculiar tooth, or the tooth requires them reciprocally ; and, thus beginning with any one, he who possesses a knowledge of the laws of organic economy would detect the whole animal. We see, for instance, very plainly, that hoofed animals must be herbivorous, since they have no means of seizing upon their prey ; we see, also, that having no other use for their fore-feet, than to support their bodies, they have no occasion for so powerfully framed a shoulder ; whence we may account for the absence of the clavicle and the acromion, and the straightness of the scapula. Not having any occasion to turn the fore-leg, their radius will lie solidly united to the ubia ; or, at least, articulated by a hinge-joint-, and not by ball and socket, as with the humerus. Their herbaceous diet will require teeth with a broad surface to crush seeds and herbs. This breadth must be irregular, and for this reason the enamel parts must be alternate with the osseous parts. This sort of surface compelling horizontal motion for triturating the food, the articulation of the jaw cannot form a hinge so close as in carnivorous animals. It must be flattened, and must correspond wiw the facing of the temporal bones. The temporal cavity, which will only contain a very small muscle, will be small and shallow.” 729. The Digestive Apparatus of Birds is similar to that of TONGUE OF BIRDS. 481 mammifers, with modifications, nevertheless, adapted to the varying habits of the different families of this class. The regimen of birds is very various : some are granivorous, some insectivorous, some piscivorous, and some carnivorous, while some subsist upon mixed food, animal and vegetable. Thus granivorous birds are often also insectivorous or piscivorous. Birds not being furnished with a dentary apparatus, the office of mastication is transferred from the mouth to the internal part of the digestive canal. The tongue sometimes serves as an instrument of prehension as well as of deglutition. A view of this organ with its appendages is given in fig. 390, where l is the tongue, h the hyoid hones, m the muscles which move them, p the pharynx, by the aid of a tube, we direct an odoriferous current towards the parts of the pituitary membrane on which the olfactory nerves are dis- tributed, the impression of the odour is strongly felt. If the current is directed to any other part of the membrane, the impression ceases. 832. Conditions of Sensibility. — It appears from these and many other circumstances that the nerves of the first pair are properly and exclusively olfactory, but that their power is inti- mately connected with other conditions of the pituitary mem- brane. Thus, to render them efficient, the secretions of which that membrane is the seat must neither be suppressed, unduly augmented, nor altered. Now, these secretions, as well as the tactile sensibility of the membrane, are placed under the influ- ence of the trifacial nerves, or those of the fifth pair, which, consequently, are endowed with functions upon which the exercise of smell indirectly depends, although they do not par- ticipate in any maimer in the transmission of the odorous impressions to the sensorium. 833. Pituitary Glands. — Each organ of sense is supplied with its proper glands, which secrete a humour peculiar to it. Thus, the organ of hearing has the ceruminous glands ; that of vision the lachrymal glands ; that of taste the salivary glands ; RELATION BETWEEN SMELL AND TASTE. 541 anti that of touch the sudoriferous glands. The olfactory organ is in like manner richly furnished with the pituitary glands, which, as then name imports, are formed in the pitui- tary membrane. These glands maintain, in a state of perma- nent humidity, the free surface of the pituitary membrane, lubricating it with the peculiar mucous fluid which they secrete. In this fluid the odorous molecules are dissolved, and, in solu- tion, afl'ect the olfactory nerves in the same manner as the gustatory nerves are affected by sapid molecules dissolved in the saliva. Although solid matter finely pulverised may excite the pituitary membrane, such effects are merely tactile, and not at all olfactory. Thus, snuff, when taken, irritates the tactile nerves, and does not necessarily affect the olfactory. 834. The Secretion of these Glands is essential, though in an indirect manner, to the exercise of the olfactory sense. When the secretion of mucus is suspended, the sense of smell ceases. 835. Close Relation between Smell and Taste. — This is admitted by all physiologists, and it has even been maintained by some that smell is nothing more than a modified taste. This opinion was, I believe, first advanced by Kant. It is rejected by Miiller, but has been recently reproduced and sustained by other physiologists, and more especially by M. Brillat-Savarin, in his work on the physiology of taste. It is contended by the partisans of this doctrine, that without the concurrence of smell there can be no real gustation. The smell and taste, according to M. Brillat-Savarin, constitute a single sense, of which he says the mouth is the laboratory and the nose the chimney. According to that physiologist, the gustatory organs of the mouth have no sense of taste except for bodies in the liquid state, to which all sapid ones which are solid are reduced by solution in the saliva ; but bodies in the gaseous or vaporous state are tasted only by the pituitous mem- brane, and their taste is called by the name of smell. 836. M. Brillat-Savarin supports this doctrine principally upon the three following facts : — 1°. That when the nasal membrane, being affected by a violent cold is deprived of its coating of mucus, the taste is entirely obliterated, no savour is perceived upon any substance which is taken into the mouth, although the tongue remains in its natural state. 2°. If we stop the nose in eating, all perception of taste becomes obscure and imperfect ; and accordingly this expedient is commonly adopted in swallowing disagreeable medicines. 542 ANIMAL PHYSICS. 3 . Tlie same effect is produced at the moment of swallowing any sub- stance, if, instead of allowing the tongue to return to its natural position it is kept pressed upon the palate of the mouth. In that case the circula- tion of air through the nasal fossae is stopped, the sense of smell sus- pended, and we are conscious of no perception of taste. 837. Odours. — Certain bodies have the property of ex- haling effluvia, by which those peculiar olfactory sensations are excited which are expressed by such terms as scent, perfume, fragrance, stench, and so on. Although bodies easily volatilised' or vaporised are placed under conditions favourable to the pro- duction of such impressions, it is neither true that such bodies are always odorous, nor that bodies not volatile or vaporisable are not so. Many gases — atmospheric air, for example — are inodorous ; and the metals, the least volatile of bodies, emit effluvia having peculiar odours. Every one is familiar with the smell of a workshop in which iron, brass, or copper is wrought. In general, the effluvia by which odorous bodies affect the smell are so subtle as entirely to escape physical and chemical analysis. A piece of tobacco, a grain of assafoetida or musk, or even a paper in which any of these substances have been wrapped, will scent a large room. It is even said that a grain of musk will continue for years to impregnate the atmosphere of a well ventilated room, without suffering any perceptible decrease of weight. To reduce the intensity of odours to numerical measure is extremely difficult, since, as has been stated, they escape all physical and chemical tests, and defy all re-agents less perfect than the organic membrane of the nose. It has, therefore, been proposed to measure at least the minor limits of then- olfactory power by dissolving odorous substances in given quan- tities of pure ail-, water, or other inodorous medium, and by gradually increasing the proportion of inodorous matter to render the solution weaker and weaker, until the odour becomes imperceptible. In this manner, odours may be grouped and tabulated according to their intensities. It is true that the same odorous solution will affect different individuals differ- ently, according to the acuteness of then- organs, but this will not prevent a relative scale being determined. The relative sensibility of the olfactory organs of different individuals may also be ascertained, by finding the relative degrees of dilution at which certain odours become imperceptible to them. Sulphuretted hydrogen, a very offensively smelling gas, is ODOROUS EFFLUVIA. 543 perceivable when a single cubic inch is mixed with half a million cubic inches of atmospheric air. 838. Bodies are more or less susceptible of being impreg- nated with odorous effluvia. Those which are most capable of retaining such effluvia are such as are most porous, and capable of receiving and retaining ah- in their interstices, or from their fluidity and affinity for air are capable of being mixed with it. Thus, clothing, wood, and water will acquire the odour of bodies to which they are severally exposed, while glass will not do so. If musk, or the attar of roses be enclosed in a glass bottle, with a close stopper, the effluvia will not penetrate or impregnate the glass, and the surrounding air will be exempt from the scent of these substances ; but if enclosed in a wooden bottle, their smell would soon be perceptible. When the air, impregnated with odorous effluvia, passes through the nasal fossae, it appears to be strained of the effluvia in the same manner as the gases of combustion pro- ceeding from the furnace of a steam-engine are strained of their heat in passing through the flues of the boiler, since no sense of odour attends the air which is expired. If, however, the nose being stopped with the fingers, odoriferous air be inspired with the mouth, and then the mouth being closed it is expired by the nose, the peculiar odour is perceived in such expiration, though not so strongly as if the air were originally inspired by the nose. This comparatively diminished intensity of the odour may arise either from the peculiar form and effect of the mechanism of expiration compared with inspiration, or from the partial dissipation of the odorous effluvia in the trachea, bronchi, and lungs. 839. Impact on Pituitary Membrane. — It would seem as though the olfactory nerves were excited by the impact of the odorous molecules, rather than by their mere contact with the pituitary membrane. When we scent a flower, we make a quick succession of inspirations, holding the flower under the nose ; and the pointer who scents the ground is observed to exercise the same respiratory action. Now, it cannot be doubted, that the air in passing through the nasal fossse deposits on the pituitary membrane, dissolved in the mucus which covers it, more or less odoriferous molecules ; and if the mere contact of these were sufficient to excite the olfactory nerves, it might be ex- pected that on stopping the nose, after a full inspiration of odorous effluvia, the sensation of the odour would be continued 544 ANIMAL PHYSICS. at least for a certain interval. This, however, is not found to Le- the case, the cessation of the consciousness of the scent being simultaneous with the completion of the act of inspiration. 840. Smell soon deadened by Excess. — No sense is so soon rendered obtuse by excitement as the smell. Every one know> how soon we become unconscious of the most agreeable per- fumes on the one hand, and of the most offensive smells on the other. Those who habitually use scents, being themselves wholly insensible to them, are reasonably enough suspected of resorting to them to disguise from others disagreeable personal emanations. This also explains why persons affected with foetid breath are unconscious of it, while those who by eructation expel offensive gases from the stomach perceive them if the mouth be closed. The former are not sensible, because respiration is continual, and the latter is perceived, because the eructation is only occasional. 841. The Nostrils, although not directly endowed with olfactory sensibility, are nevertheless subservient to the sense of smell by giving such a direction to the respiratory current, as to propel it along the walls of the fossae. In the case of persons who have lost the nose, the current, following the shortest route, passes chiefly along the floor of the fossae, coming but little in contact with the walls, and thus escaping, in a great degree, the pituitary membrane, which is the more especial organ of smell. In such cases, the olfactory functions are re-established by an artificial nose. When we desire to excite in a great degree the sense of smell, we close the mouth, so as to draw the respiratory current exclu- sively through the nose. On the contrary, to avoid a disagreeable odour, we stop the nostrils and respire exclu- sively through the mouth. The same purpose, however, may be in a great degree attained, by keeping the mouth open with- out stopping the nostrils, because by so doing respiration takes place chiefly by the mouth, and very little by the nose. The air in the nasal fossae, therefore, being scarcely changed, there is but little exercise of the sense of smell. 842. Different Susceptibility of SmelL — Different indivi- duals are differently affected by particular odours, according to the varying condition and susceptibility of their nervous system. Many odours perceivable by some are totally unperceived hj others ; such, for example, as the scent of violets, mignonette, SMELL OF ANIMALS. 545 and some other flowers. This difference of susceptibility is still more striking when man is compared with animals, many of which are strongly sensible of scents of which we are totally unconscious. Thus the dog tracks his master over ground upon which other individuals have subsequently passed, being, there- fore, not only capable of perceiving the general scent of man, but of distinguishing the special scent of one particular person. Odours which are agreeable to some are offensive to others. Musk and assafcetida are examples of these. The repugnance of some persons to musk is such as to produce syncope. Manjr perfumes agreeable to some produce megrim, nausea, and swooning in others. 843. Subjective Olfactory Sensations are less common and less understood than the coiTesponding illusions of sight and hearing. They are rare in some persons, but not uncommon with those afflicted with mental derangement, having their origin evidently in cerebral disturbance. Thus, insane patients often complain that foetid or faecal matter has been mixed with their food. 844. The Direction of Odorous Objects cannot be at all determined by the organ of smell, as that of visible and audible objects is by the organs of seeing and hearing. When odours are brought within the perception of the sense by currents of air, the directions in which they lie are determined not by the olfactory organ, but are judged of by the mind, which imputes their transport to the aerial current, and identifies their direc- tion with the direction of that current. 845. The Olfactory Sense of Inferior Animals. Mammifers have generally this sense more developed than man. In ru- minants, carnivora, and rodents, the pituitary membrane is relatively much more extensive, and has a structure which gives it a greater surface within a given space. 846. Birds do not exhibit much development of the olfactory organs. The olfactory lobes of the cerebrum, from which the nerves of smell proceed, are, however, considerably developed. This is observable more especially in predaceous and piscivorous birds. Notwithstanding this, birds do not seem to be endowed with great delicacy of smell, discovering their prey in general by the sharpness of their vision. N N 54G ANIMAL PHYSICS. 847. Reptiles have nasal cavities, consisting of two canals, opening outwards by nostrils and communicating with the mouth by two holes in the vault of the palate. In the case of naked reptiles the nasal canals are merely lined with mucous membrane. In that of scaly reptiles, the turbinate bones are more or less developed. The olfactory nerves have, in general, considerable volume, and are conducted to the nostrils by a special bony and cartilaginous canal, formed for them in the bones of the skull. 848. Pishes, inhabiting the water, are supplied with an olfac- tory apparatus suitable to that fluid, and consisting of two small cavities in a cul-de-sac, opening outwards by two nostrils. The bottom of these sacs generally consists of folds, sometimes grouped radially with a central part, and sometimes arranged like the leaves of a book. These sacs receive filaments of the olfactory nerve proceeding from the cerebral lobe. The water which imparts odours to the olfactory membrane cannot be so freely and frequently renewed as in the case of animals which respire in the air, for there is here no continuous current, pro- perly speaking, the water being ejected from the opening at which it is admitted. The sense of smell in this class must therefore be very imperfect. 849. The Annulata, Mollusca. and Radiata. appear to be endowed with no special organ of smell. It is certain, never- theless, that some of these, and more particularly insects, are not altogether deprived of this sense. Flies, bees, and gnats, are attracted from a distance by honey, sugar, meat, and other objects which supply their nourishment. Some physiologists think that it is in the antennas or tentacula that the faculty of smell resides. Cuvier, however, was of opinion that the olfactory sense of insects was exercised by the stigmata placed at the embouchures of their tracheae, or organs of respiration. SENSE OF TASTE. 547 CHAPTER XIY. TASTE. 850. The Tongue. — Although the tongue be the principal organ of taste, other parts of the mouth co-operate directly and indirectly in the exercise of that sense. Neither are all the parts of the tongue itself sensible of gustatory impres- sions, nor are those which are so, equally sensitive to them. The mucous membrane of the tongue is supplied with nume- rous papillae unequally distributed, having very different forms and, apparently, gustatory and tactile properties with corre- sponding differences. It is also richly supplied with blood- vessels and nerves, and is composed of a mass of muscles and nerves, which give it that infinite mobility so necessary to speech, and the processes of mastication and insalivation. 851. Various Experiments have been made to determine the parts of the mouth which are more or less endowed with the gustatory sense, but hitherto no very definite results have been obtained. Such researches are attended with many difficulties. The organ can only be excited by sapid matter in a state of solution. If the substance under experiment be solid, it must therefore be soluble in the saliva, and when thus liquified, it has a tendency to diffuse itself over parts of the mouth to which the experiment is not addressed. Expedients have been contrived therefore to localise the action of the sapid matter. Messrs. Pemiere, Panizza, and others, used small sponges impregnated with the sapid solution, and attached to the extremities of whalebone rods. They also, in certain cases, supported the solution in capillary tubes, applying their extremities to the part under trial. Messrs. Guyot and Admyrault enclosed the tongue in a glove of softened parchment, in which a hole was made corresponding in its position to the part whose gustatory sensibility was to be tried. In all such arrangements, however, the taste is exercised under abnormal and unnatural conditions, and the results, whatever they may be, are unsatisfactory and inconclusive. In n n 2 548 ANIMAL PHYSICS. natiu’al gustation the tongue is applied with a certain degree of friction against the palate, and the universal consciousness of the concurrence of this latter part in the exercise of taste is in- dicated by the familiar use of the term palatable. It seems to be generally agreed that the principal seat of the sensation of taste is near the root, or what anatomists call the base of the tongue, that is, the part of the tongue which is attached to the posterior pai-t of the mouth; but it is also demon- strated that the upper part of the pharynx and the lower part of the pillars of the veil of the palate participate in the gusta- tory sensibility. Most physiologists deny to the lips, gums, cheeks, dorsal, or upper surface of the tongue, and the vault of the palate any sapid sensibility. Whether the point of the tongue, its borders, its inferior surface, or the membranous part of the vault of the palate are sensible to taste, is disputed. In all investigations directed to the solution of this class of physiological problems, it must never be forgotten that alkaline, acid, astringent, and acrid principles affect the tactile and not the gustatory sensibility. Sapid substances of a saccharine, salt, or bitter flavour produce no sensation of taste when applied exclusively to the point, borders, or inferior surface of the tongue. If these parts be sensible to taste at all, it can only be to those whose action upon the gustatory nerves is one of extreme intensity. When a sapid substance is deposited upon the parts of the tongue which are admitted to be endowed with gustatory sensi- bility, the taste is not excited in any considerable degree until the mouth has been closed, and the tongue pressed and rubbed against the palate. It does not necessarily follow that in this case the palate itself is sensible to the taste ; but the applica- tion of the tongue compresses the gustatory papillre, and increases the action of the sapid matter upon them by friction, and thus excites in a greater degree, the sensation of taste. 852. In deglutition and mastication, the sapid principles of the food are made to act upon the various sensitive parts of the mouth, so as to excite gustation. Mastication, by the repeated friction which attends it, and the incessant play of the tongue, and all the softer parts, especially stimulates the sense of taste. The aliment being dissolved in the saliva is also reduced to a condition more favourable to the excitement of gustation. A long-continued and repeated contact of the sapid mattei is necessary to the full developmcut of the sense of taste. Tin1 wine-taster judges of the quality of the liquor under trial In SEAT OF GUSTATION. 549 moving it round till parts of his mouth repeatedly, and allow- ing it to remain in contact with those parts most strongly endowed with gustatory sensibility before he swallows or ejects it. 853. Limit of Gustatory Sensibility. — The organ of taste is infinitely less sensitive than that of smell, as may be rendered evident by comparing the relative strength of the solutions which affect the two senses. If a grain of sugar be dissolved in a hundred grains of pure water, no degree of sweetness will be perceived in the solution. If a grain of common salt be dissolved in two hundred grains of water, the solution will be also destitute of all sensible saltness. The solutions of certain bitter or sweet principles, such as strychniue or salt of silver, are sensible to taste, it is true, at a very inferior degree of strength ; but, even in these, the propor- tion is infinitely below those by which, as already explained, the sense of smell is strongly affected. Colocynth is, as is well known, a strong bitter ; but if a grain of it l>e infused in 5000 grains of water, its peculiar flavour will not be perceived. 854. Delicacy of Taste varies.— The sense of taste varies extremely in different individuals, as may be seen in the different degrees of enjoyment they receive from the pleasures of the table. Some individuals seem almost indifferent to the nature and qualities of their aliment. Many are so regardless of this, that they scarcely know the difference between beef and mutton. Others, on the contrary, regard the act of eating as one of the chief pleasures of life. Such immoderate enjoyment of the table must not, however, be exclusively ascribed to the greater sensibility of the taste, since much, no doubt, depends upon the delicacy of the olfactory sense which, as already explained, is so closely connected with the sense of taste. 855. Experiments which are more or less familiar to every one, demonstrate this close connection of the two senses. We are enabled with great facility to perceive the quality — good or bad — of various kinds of food, such as meat, fish, butter, milk, or bread ; but if these be tasted and masticated, with the nose stopped and the eyes bandaged, no perception of the goodness or badness of their qualities will be produced. In such cases we do not easily distinguish, for example, wine from water, and most aliments lose their peculiar taste. It is only very strong gustatory qualities, such as those of salt and sugar, that are perceptible. 850. Papillary Structure of the Tongue. — Papilla) (780), found on all parts of the tegumentary integument of the 550 ANIMAL PHYSIC'S. body, internal as well as external, being the more immediat - seat of sensibility, it may be expected that the mucous mem- brane of the tongue, endowed as it is in so eminent a degree with both tactile and gustatory sensibility, should exhibit a papillary structure of peculiar richness. Anatomical observation, more especially when seconded by the powers of the microscope, fully verifies this anticipation. All parts of the surface of the tongue are thickly covered with papillse, but these are not uniformly distributed, nor are they similar in form or magnitude. 857. Dorsal Surface of Tongue. — It is on the superior sur- face, or dorsum as the anatomists call it, that the papilla; prevail in the greatest numbers. A view of the dorsum is given in fig. 415, from the work of Professor Sappey, which is by far the most accurate and elaborate representation of the organ which I have seen. That anatomist classes the lingual papillae in four orders called Calyciform, Fungiform, Corolliform, and Hemispherical. 858. 1°. Calyeiform Papillae. — The first in order, and largest in their dimensions of the lingual papilla', are disposed in the form of the legs of the letter A upon the upper surface of the tongue, the point of junction of the legs being directed backward, and holding a position in the middle of the breadth of the tongue, at a part about three-fourths of its entire length, from the anterior extremity, or tip. These are shown converging from 415 1 to 415 3. The number of these papilla of the first order, varies from eight to fourteen or fifteen. Each of these has the form called in geometry that of a truncated cone, that is, a cone with its point cut off. The smaller end of the cone forms the base of the papilla, and the greater end is level with the general surface of the tongue. The papilla is implanted in a eup-like cavity surrounded with a circular furrow or trench, outside which is a circular ridge rising to the level of the upper surface of the papilla. The whole resembles that of the crater of a volcano, if its central cone be conceived to be inserted so as to stand upon its blunted apex. This class of papillae has been called by some anatomists vallafm, or circumvallatcc, from the circumstance of being surrounded by a trench. They are also called calyciform, from the circumstance of standing within a cup. 859. Foramen Caecum. — At the point of the A is a part (415 s) called the foramen ccecum, described by the older anatomists as the excretory duct of a salivary gland, and by Quain, Wilson, and other English elementary writers, as the embouchures of several mucous glands or follicles. Some foreign anatomists, however, and Professor Sappey in particular, affirm this to be au error, and describe it as the largest of the calyciform papilla', being sometimes single, but often consisting of two or more papilla- included within a single calyx. LINGUAL PAPILLiE 551 860. 2°. Fungiform Papillae. — In front of the papilke here described, and occupying the middle third of the length of the tongue, are foundnumerous others, much less voluminous, but more numerous and more closely packed together, like the pile of velvet. These stand perpendicular to the lingual surface, and are not inclined backwards, as was maintained by Malpighi. They are generally arranged in parallel lines emerging from the 552 ANIMAL PHYSICS. axis of the tongue as branches emerge from the stem of a leaf. The*; papillse have generally a form resembling that of a mushroom, being expanded at the summit and contracted at the base, whence they have derived the name of f ungiform. Their magnitude is greater than that of other papillae by which they are surrounded, but very inferior to that of the calyciform above described. These fungiform papillae are shown in fig. 415 3. 3°. The Corolliform Papillae called also conical, cylindrical 0r fili- form, by most anatomists, are smaller than the fungiform, and cover all that part of the tongue in front of the calyciform with a sort of tufted grass- through which the particles of sapid liquids diffuse themselves. These papilla: have very various dimensions and forms. Some are filiform, having a uniform diameter ; others, being larger at the base, are conical. Other- are more or less oval in their transverse section ; others are quite flat : and, in fine, some are prismatic, triangular, pyramidal, &c. 861. 4°. Hemispherical Papillae. — These are of extreme minutene?-. not exceeding those of the last phalanges of the fingers. Their number is so great as to be incapable of any calculation. They prevail in immense numbers in the intervals between the fungiform and corolliform papilla?, and at the bottom of the trenches which surround these. 862. Distribution of Papillae.— It appears from the obser- vations of Professor Sappey that these foxu- orders of papillae, although to a certain extent mingled together, nevertheless prevail in greatest numbers at particular parts of the tongue. Thus the calyciform are limited to that part of the upper surface of the tongue whose distance from the anterior extremity is two-thirds of the entire length. The fungiform are scattered over the two anterior thirds of the upper surface, occupying more especially the tip and the anterior half of the border of the organ. The corolliform papilke prevail in nearly the same regions, but exist also in a little group behind the calyciform papillse. The hemispherical papillse, or those of the fourth order and of the greatest minuteness, prevail principally on the inferior surface, lateral parts, and the posterior third of the upper surface. 863. Microscopic Appearance. — All these papillae which, when viewed with the naked eye or even with a low magni- fying power, appear so different, exhibit in the microscope a remarkable analogy of structure. The calyciform, fungiform, and corolliform papillae are, according to the observations of Professor Sappey, nothing more than different agglomerations of hemispherical papillae accumulated at the point in question. This explains why sapid impressions are so intense upon that part of the dorsal surface which is at a distance from the point LINGUAL PAPILL/E. 553 of the tongue equal to two-thirds of its length, the elementary papillae there forming groups so remarkably voluminous. One of the calyciform papillae found in the region of the tongue, as examined by Professor Sappey, was magnified twenty times in its linear dimensions. It appeared from this, that not only that which appears to the naked eye as a large central papilla is composed of innumerable minute elementary papillae, but also that the surrounding circular ridge is similarly constituted. 864. According to the same authority, the sensibility of the tongue upon the point and borders is also explained by the groups of papillae of the second order, called fungiform, which are accumulated there. The sensibility of the middle part of the upper surface between the region of calyciform papillae and the extremity, and the complete insensibility of that part which is situated behind the calyciform papillae, is explained by the fact that in the former part papillae of the third order only prevail mingled with but few groups of the second and inferior orders, and that in the latter region the only papillae found are those of the lowest order. Like considerations explain the absence of gustatory sensibility in the inferior part of the tongue. The relation thus established, observes Professor Sappey, between the number of elementary papillae found at any given point of the lingual surface and the degree of special sensibility with which that part is endowed, is so true that we find it not less manifest upon another part of the mouth endowed, like the tongue, with the property of being affected by sapid substances. It has been demonstrated by experiment that the anterior part of the vault of the palate possesses gustatory sensibility in a remarkable degree ; and we find, in accordance with this principle, upon this point, and upon this point only, a voluminous calyciform papilla, situate upon the median line behind the middle incisives, and round it a crowd of tubercles in ridges, bristling with elementary papilke. 865. Nerves of the Tongue. — The mucous membrane of the tongue which, as has been observed, is the seat of gustative sensibility, is supplied with nerves from two principal sources : first, from the trigeminal or trifacial nerves, being those of the fifth pair, the origin of which is shown at 244 2;; and, secondly, from the glosso-pharyngeal, or ninth pair, the origin of which is shown at 244 27. According to Professor Sappey, filaments are also sent to this membrane by the pncumo-gastric, or tenth pair, the origin of which is shown at 244 32. The trigeminal sends a voluminous branch called the lingual nerve into the lower jaw, the course of which is shown at 270 3 and 271 l4. This branch entering the tongue, as shown at 416 k, ramifies through the 554 ANIMAL PHYSICS. organ, its filaments terminating in the mucous membrane of the superior surface. It supplies all that part of the tongue which extends from the point over three-fifths of its length. The posterior two-fifths are supplied with nervous filaments by the principal branch of the glosso-pbaiyngeal, the course of which on entering the tongue is shown at 416 *. The pneumo-gastric nerve sends to the tongue some small ramifications, which proceed from the superior laryngeal branch. The ramifications of this nerve which supply the larynx are shown at 2716 and 271 ', but do not appear in 416. They pass into the tongue, losing themselves in that part of the mucous membrane which is situated immediately before the epiglottis, which is not endowed with sapid sensibility. This nerve, Fig. 416. NERVES AND MUSCLES OF THE TONGUE (Beclard). a. Section of the bone of the lower jaw. b. Dorsal or superior surface of the tongue. c. Vertical section of the tongue. d. Genio glossal muscle. e. Genio-hyo-glossal muscle. /. Hyo-glossal muscle. g. Stylo-glossal muscle. h. Hyoid bone. k. Lingual nerve. i. Glosso-pliaryugeal nerve, m. Hypoglossal nerve. therefore, cannot properly be considered as a gustatory nerve. Tire motor nerve of the tongue is the hypoglossal (416 m), which spreads its ramifica- tions exclusively through the lingual muscles. The lingual (416 k) and glosso-pharyngeal (416 *) are therefore, accord- ing to some authorities, the nerves of gustatory sensibility. The former, after traversing the muscles, terminates in the papilla; situated in front of the lingual A (858). The latter, after having spread itself under the LINGUAL NERVES. 555 mucous membrane, terminates in the calyciform papillae and those of the other classes with which these are surrounded. Both these nerves impart, also, tactile sensibility to the parts over which they are distributed. Whether each individual fibre be endowed at once with the tactile and the gustatory sensibility, or whether the nerve, having these sensibilities, consists of two distinct fibres in juxtaposition, one gustatory and the other tactile, physiologists have not decided. The latter hypothesis, how- ever, appears to have been more generally accepted. To establish it by absolute demonstration, it would be necessary to isolate the two sensibili- ties upon the same trunk by paralysing, for example, the gustatory and preserving the tactile ; but the two fibres, if such exist, endowed with these distinct properties, are so intimately connected that their separation has not hitherto been found possible. According to Professor Berard, how- ever, nature has in some cases performed this experiment. That anatomist records six cases of the paralysis of the tactile sensibility, the gustatory remaining unimpaired. Upon the hypothesis of the double sensibility in a single fibre, it would be impossible to explain these phenomena, whereas their explanation is easy and obvious upon the other supposition. * A controversy has prevailed among physiologists as to whether the lingual branch of the fifth pah or the glosso-pharyngeal nerve be the nerve of taste. Wagner, Panizza, Valentin, and Bruns place the principal seat of gustatory sensibility in the glosso-pharyngeal nerve, while Muller, Komfeld, and Gwilt give it to the lingual branch of the fifth. The latter physiologists consider Valentin’s experiments proving the glosso-pharyn- geal to be the principal gustatory nerve as inconclusive, inasmuch as the animal in which it was divided recovered its taste within a fortnight, a period so short as to render it probable that the sense was never lost. They regard Dr. Alcock’s experiments also as inconclusive. That physiologist inferred that the sense of taste was seated not only in the glosso-pharyngeal and lingual branch of the fifth, but also in the palatine branches of the fifth. It appears certain, according to Muller, as well from his own experiments as from those of'Magendie, Gwilt, Komfeld, Parry, Bishop, and Romberg, that the larger branch of the fifth is the principal gustatory nerve, although the absence of gustatory sensibility from the glosso-pharyngeal at the posterior part of the tongue and in the foss£e is not proved. 866. Terminal Form of Lingual Nerves. — Whether the nerves of gustation terminate in what anatomists call a free extremity in the various papilke, or are looped in the manner formerly described (30), has not been determined. Eacli of these two hypotheses has had its partisans. Burdach, who favours the loop form, says, that he has observed it in the nerves upon the point of the tongue of a frog ; and Valentin appears to have arrived at the same result by the application of caustic potash. Professor Sappey has made numerous experi- ments upon the human tongue to determine this point, but has hitherto failed to discover any appearance which gives decided Berard, Traits de Physiologic. Sappey, Traite d’ Anatomic, vol. ii. p. 764. 556 ANIMAL PHYSICS. countenance to the one or the other hypothesis. He has only found that the nerve occupies the smallest imaginable space in the interior of the papillae, nearly the entire volume of which consists of mucous membrane, cellular tissue, arteries, veins, and lymphatics.* 867., Vascular Apparatus of Mucous Membrane. — The lingual membrane is richly supplied with arteries, veins, and lymphatics. The ar-teries consist of innumerable ramifications proceeding from the lingual artery which supplies the subjacent muscles. The veins are, however, independent of those of the muscles, and consist of three groups — one superior or medial, and two inferior or lateral. 868. From all that has been stated, it will be understood that much difference prevails among physiologists, and that much remains still to be discovered respecting the exact seat of gustatory sensibility. Thus Messrs. Panizza and Valentin deny this sensibility to the point of the tongue, and localise it upon its upper surface and the superior part of the pharynx. They consequently regard the glosso-pharyngeal nerve as the special nerve of taste, and impute to the lingual nerve nothing more than the tactile sensibility, which is admitted on all hands to be eminently delicate at or near the point of the tongue, where the filaments of the lingual nerve are most numerously distributed. The gustatory sensibility of the point of the tongue is regarded by most physiologists as very doubtful, and it is con- tended that the great delicacy of the tactile sensibility has led many erroneously to impute the other special sense to it. It is certain that many sapid substances affect this part of the tongue, but in a manner rather tactile than gustatory. 869. M. Panizza having divided the glosso-pharyngeal nerve of dogs, found that complete loss of taste ensued. A dog thus treated ate indifferently meat in its natural state and meat impregnated with colocynth, and also swallowed drinks with the greatest facility in which the same bitter principle was dis- solved. Now it is known that the healthy animal entertains an in surmoun table disgust to colocynth. 870. Admitting that the glosso-pharyngeal nerve is exclu- sively endowed with gustatory sensibility, it must also be admitted that it possesses the tactile sensibility, whether both * Sappey, Traitt? d’Auatomie, p. 765. SEAT OF GUSTATION. 557 these properties be ascribed to the same fibres, or, according to the other hypothesis, the nervous cords be assumed to consist of different fibres endowed with the two sensitive principles. If this double function therefore be ascribed to the glosso-pliaryn- geal nerve some countenance is given to this supposition, which would ascribe a like double function to the lingual nerve. 871. The Sense of Taste of Inferior Animals is much less developed than with man. It is not the sense of taste, but generally that of smell, which guides them in the choice of their food, since this choice always precedes its prehension. The uncertainty which exists as to the precise seat of the sense of taste in man is still greater in the case of inferior animals. It is probable that the superior part of the digestive cavities, such as the pharynx;, which, in man, shares with the tongue the property of transmitting gustatory impressions, presides alone over the perceptions of taste in most of the animals which want the tongue, and even in those in which that organ, being converted into an instrument of prehension, is horny or supplied with a sort of dental appendages. 872. The Tongue of Mammifers in general resembles that of man. The tongue of the dog is covered with soft and numerous papillae. That of the larger ruminants, of the cat, and animals of the same class, is supplied with papillae inclined backwards and inclosed in a horny sheath more or less thick. When a ruminant browses, these papillae fix upon the tongue the grass which the animal seizes. When a carnivorous animal licks the prey which it has torn, the rugged surface of the tongue tends to make the blood upon which it feeds issue forth. Other mammifers, such as ant-eaters, echidnae, cetaceas, and so on, have a tongue nearly deprived of papillae, and therefore destitute of gustatory sensibility. 873. Birds have a very obtuse sense of taste. They swallow thSir food almost without mastication. The tongue is generally hard and semi-cartilaginous, especially near the point. Grani- vorous birds are particularly remarkable in this respect ; but birds of prey, which live on flesh, arc supplied with a more fleshy tongue. 874. Reptiles have in some cases a thick fleshy tongue, but 558 ANIMAL PHYSICS. it is more generally thin, protractile, often bifid, and constitutes rather an organ of prehension than of taste. 875. Pishes have a tongue scarcely more than rudimentary. With many of them it is almost immovable, and furnished, like most of the other parts of the buccal cavity, with homy or bony appendages which enable the animal to retain its prey. If fishes be endowed with taste at all, its seat must be limited to the superior part of the digestive canal. 876. Invertebrata are nearly in the same case, having nothing which resembles a tongue. If they possess the gusta- tory sense, which insects probably do, its seat must be the soft parts of the mouth, the suckers, or the proboscis. ORGAN OF VISION. 559 CHAPTER XV. VISION. 877. Of all tlie organs of sense, the eye is that to which we are most deeply indebted. It opens to us the widest and most varied range of observation. The pleasures and advantages it affords, directly and indirectly, have neither cessation nor bounds. It guides our steps through the world we inhabit, and invests us with a space-penetrating power to which there is no practical limit. Although vision, strictly speaking, is only cognisant of light and colour, yet, from the habitual comparison of the connection of these with the forms of bodies, as ascertained by touch, we acquire the greatest facility, promptitude, and precision in recognising, by sight alone, the forms, magnitudes, motions, distances, and relative positions, not only of all the objects which immediately surround us, but also of innumerable others which are altogether inaccessible. This sphere of observation, however, vast as it is, forms but a small part of the power conferred by the eye. We have, besides, the inestimable advantages attending the ability it bestows upon us to acquire knowledge through books, to converse with and be instructed by the learned, the wise, and the virtuous of all ages ; and although those who have the misfortune to be deprived of vision can, to some extent, replace it by the ear, aided by the eye of another, yet this and all like expedients supply results infinitely small and insignificant compared with those obtained by the organ of vision itself. 878. Apart from its uses, the eye itself is a most interesting and instructive object of study. It presents, beyond compa- rison, the most beautiful example of design to be found in the animal economy. Nowhere can we find so remarkable an adap- tation of means to an end — means consisting of the most perfect combination of scientific principles, and an end manifesting a will directed by boundless beneficence. 879. The Visual Apparatus in man and the higher animals consists of three chief parts : 1st, the organ of sense itself ; SCO ANIMAL PHYSICS, 2nd, the parts by which the impressions upon it are transmitted to the brain ; 3rd, the parts by which it is moved and protects ' , and its efficiency maintained. 880. Structure of the Eye. — In the human race the organ of vision consists of two hollow spheres, each about an inch in diameter, filled with certain transparent liquids, and deposited in cavities of suitable magnitude and form in the upper part of the front of the skull, on each side of the nose. These cavities axe lined, with soft matter, serving as a cushion for the protection of the eyeballs, which can move freely in them, the surfaced- being lubricated by fluids secreted in surrounding glands. The organs are further protected from external injury by the projecting bones of the fore- head above, forming the brows, the bones of the temples on the outside, those of the cheeks below, and those of the nose on the inside. The eyeballs are constructed so as to form upon the posterior part of the inside surface of each of them an optical picture of every external object placed before them. They are nearly spherical, and the transparent fluids called humours, which fill the internal cavities, are inclosed in a triple membranous envelope. The external coat, called the sclerotic, upon which the maintenance of the form of the eye chiefly depends, is an opaque, tough structure, com- posed of bundles of strong white fibres, interlacing each other in all directions. It covers about four-fifths of the eyeball, leaving two circular openings, — a large one in front, covered by a transparent convex piece of nearly uniform thickness, called the cornea, and a smaller one behind, the embouchure of the optic nerve, which, proceeding backwards and upwards, and passing through foramina in the bones of the skull, terminates in the brain. It is by this nerve that the impressions of external objects are transmitted to the seat of perception. 881. The Cornea, closely united at its edge with the correspond;: _ edge of the circular opening in the sclerotic, is slightly elliptical, its hori- zontal being rather longer than its vertical diameter. Its external surfr. e is more convex than that of the sclerotic, so that it forms a segment of i sphere smaller than that of the general surface of the eyeball. It. there u i p \V Fig. 417. COATS OP THE EYE. 561 fore projects outwards in front of the eye, rendering that axis of the eye which passes through its centre a little longer than the diameters, at right angles to it. Being of nearly uniform thickness, the concavity of its inner surface corresponds with the convexity of its outer, and gives the whole the form of a watch-glass, or a concavo-convex lens, whose surfaces have equal radii. ..... . In looking at an open eye, that part of the sclerotic which is uncovered is what is popularly called the white of the eye , and the cornea covers the coloured part. , -cue A front view of the eyes and surrounding parts is shown in ng. 417 ; a section of them, made by a horizontal plane through the line A b passing through the centre of the front of the eyeballs, being shown in fig. 418. The sclerotic is shown at c d f e, and the cornea at d g f. A line, m t, drawn through the centre of the cornea and the centre of the eyeball, is called the optic axis, and the embouchure, c e, of the optic nerve lies at the distance of about the tenth of an inch from this axis, between it and the nose. The optic nerves, r, therefore, issuing from the two eyeballs at the corners, beside and behind the nose, proceed in a con- verging direction to the brain, as shown in fig. 418. The connection of the seat of vision with the brain by the optic nerve is shown in fig. 419, where s is the eyeball, the end of the optic nerve enter- ing its posterior part, and receding backwards from thence to the brain. The other nerves here represented direct the movements of the eye. 882. Choroid. — Within the sclerotic, and in contact with it, is the second coat, called the choroid, n (fig. 418), a dark-coloured vascular membrane, having openings before and behind corresponding with the cornea and optic nerve, similar exactly to those of the sclerotic. 883. Retina. — Within this is the third membranous coating (fig. 420), called the retina, which is, in fact, the continuation of the fibres of the optic nerve spreading over the chief part of the internal surface of the eye- ball. It is a delicate, pulpy, and perfectly transparent membrane, spread over all the posterior and lateral parts, terminating near the margin of the frontal opening covered by the cornea already described. T i. Fig. 418. 0 0 562 ANIMAL PHYSICS. SSI. Crystalline. — As the frontal opening of the sclerotica r- ei :-r i by the cornea, that of the choroid which corresponds with it in position is ' i v l Fig. 419. closed by a transparent double convex piece, called the crystalline lens, the axis of which coincides exactly with the optic axis, and is con- sequently concentric with the cornea. It is fixed by means of a series of converging folds of that membrane, called the ciliary processes. The annular surface formed by these processes, and the crystalline lens which they surround and support, form the posterior side of a compartment in the front of the eyeball, separated completely from the larger compartment behind the crystalline lens. This arrangement will be more clearly com- prehended by the enlarged section of the front of the eye given in fig. 420, where 2 is the sclerotica, 3 the cornea, b the crystalline lens, and 6 the ciliary processes. 885. The Iris is a thin flat annular diaphragm, the section of which is shown at 7, dividing the space between the crystalline lens and the cornea unequally into two parts, called the anterior chamber, a, and the posterior chamber, d. The external or anterior surface of the iris is coloured blue, black, or hazel, differently in different eyes, and is the part which, seen through the transparent cornea, gives the characteristic colour to the eye. 886. The Pupil is a circular opening surrounded by the iris, through which the light received through the cornea is transmitted to the crystalline lens. By this means there is admitted to the crystalline a pencil of rays whose external limits are determined by the edges of the iris. The posterior HUMOURS OF THE EYE. 563 surface of the iris is covered by a black pigment, contained in a thin transparent membrane, called the uvea. In fig. 421 a view of the ciliary processes (1) which surround and sup- port the crystalline lens is given. That lens, however, being supposed to be removed, the converging folds of which they consist are shown, and the iris (2) is seen by its dark posterior surface through the space filled by the crystalline, with the pupil (3) in its centre. When seen from the front, the pupil appears as a black circular spot p (fig. 417), surrounded by the coloured ring of the iris, because every part of the interior of the eye which could be visible through it is coloured black. 887. The Aqueous Humour fills the compartment of the eye between the cornea and crystalline, and, as its name implies, is a watery fluid, holding in solution very minute quantities of albumen and common salt. It is separated from the cornea by an extremely thin transparent membrane, shown at 420, n, called the membrane of the aqueous humour , which, however, is represented much too thick in the figure. Fig- 420. Fig. 421. The Crystalline Lens (420, b) is enclosed in a transparent capsule, and consists of transparent matter, which increases in density and in its refractive power, proceeding from its external surface inwards, and from its edges to its centre. 888. The Vitreous Humour fills the posterior compartment of the eye (420, cc) behind the crystalline, and constitutes by far the largest part 0 o 2 5G4 ANIMAL PHYSICS. of the internal cavity. This is not in immediate contact with the retina, being enclosed in a fine transparent membrane called the hyaloid (420, *). 889. The Eyelids are not in immediate contact with the s :ler or the cornea. A fine mucous membrane, called the conjunctiva, which lines their inner surface, is reflected over the fore part of the sclerotic and the anterior surface of the cornea. A part of this membrane is shown in section at 420, '. 890. The Eyebrows and other Accessories are provided for the protection and preservation of the organ of vision. The eyebrows across the edge of the projecting part of the forehead catch the sweat descending from above, and prevent it from falling on the eyes, and aid in shading the eyes from too intense light from above. The eyelids are movable screens, made so as to cover the eye or leave it exposed, as occasion may require. Glands are provided, by which all the parts which move in contact one with another are kept constantly lubricated. 891. Numerical Data of the Structure. — The principal numerical data connected with this organ : — following are the lOOths of Inch. Radius of sclerotic coating ..... . 39 to 43 Radius of cornea ...... . . 2S 32 External diameter of iris ..... • « „ 47 Diameter of pupil ..... . . 12 „ 2S Thickness of cornea ...... . 4 Distance of pupil from centre of cornea . . S Distance of pupil from centre of crystalline . 4 Radius of anterior surface of crystalline . . 2S „ 39 Radius of posterior surface of crystalline . . 20 „ 24 Diameter of crystalline ..... . . 39 Thickness of do. ...... . 20 Length of optic axis ..... . . S7 „ 95 Index of refraction from air into aqueous humour 1 3366 Index of refraction from air into vitreous humour Index of refraction from air into crystalline humour : — . . 1-3394 At the surface ..... . 1*3767 At the centre ..... . . 1-3990 At the mean ...... 1-3SS9 Index of refraction from aqueous humour to crystalline humour : — At the surface . . . . . . . 1 '0466 At the mean ....... 1'0353 Index of refraction from vitreous humour to crystalline humour : — At the surface . . . . . . . 1 '0445 At the mean . . .... 1 "OSS2 According to Sir D. Brewster, who has supplied the preceding indices of refraction, the focal length of the crystalline is 1'73 inches. 892. The Motor Apparatus, by means of which the optic axis can be directed at will within definite limits to surrounding objects, consists of muscles inserted at various points of the sclerotica, and having their origin in the bones of the socket. These muscles are acted upon by a corresponding system of nerves. The motions thus imparted to the eyeball are facilitated by a MOTOR APPARATUS OF THE EYE. 565 lubricating fluid secreted by a gland, called the lachrymal gland, placed over the eyeball. This fluid is continually spread upon the siu-face of the sclerotic, by the motion of the eyelids in winking. In fig. 422 the motor muscles ancl the lachrymal gland of the right eye are shown by the removal of the lateral bony parts of the external side of the socket. 3 Fig. 422. MOTOR MUSCLES AND LACHRYMAL GLAND OF RIGHT EYE (Sappey). 1. Muscle which raises the eyelid. The tendinous expansion of this muscle has been cut away to display the palpebral portion of the lachrymal gland covered by it. 2. if. which directs the optic axis upwards. 3. M. which directs the axis outwards. 4. M. which directs the axis downwards. 5. M. of unascertained use. 6. Orbital part of lachrymal gland. 7. Palpebral part traversed by four ducts of orbital part and sending into these small ducts or canalicules. 8. 8. Accessory ducts proceeding exclusively from tho superior border of tho palpebral part. 9. Another accessory duct with three lobules. In fig. 423, the lachrymal gland, the eyelid, and accessories are removed in order to display the parts they conceal. 893. The Limits of the Play of the Eyeball are as follows : — The optic axis can turn in the horizontal plane through an angle of 00° towards the nose, and 90° outwards, giving an 566 ANIMAL PHYSICS. entire horizontal play of 150°. In the vertical direction it is Fig. 423. The insertion of the three muscles marked 2, 3, 4, in fig. 422, but here marked 2, 5, 6, upon the sclerotic are apparent. A fourth muscle inserted on the inside of the eyeball, by which the optic axis is directed inwards, is concealed. These four are called the straight or recti muscles. The muscles 423i, and 42310, are called the superior and inferior oblique. Physiologists arc not agreed as to their use, but 4232, is supposed to move the axis upwards and inwards, and 423 i0, downwards and inwards. 4231, is the muscle which elevates the eyebrow. 4233, the pulley of the M. 4232. 42311, optic nerve. 4231-, section of the jawbone. 42313, upper jaw. capable of turning through an angle of 50° upwards and 70° downwards, giving a total vertical play of 1 20°. 894. Ocular Image. — The structure of the eye being thus understood, it will be easy to explain the effect produced within it by luminous or illuminated objects placed before it. Let us suppose a pencil of light proceeding from any luminous object, such as the sun, incident upon that part of the eyeball which is left, un- covered by the open eyelids. That part of the pencil which falls upon the white of the eye, W (fig. 417), is irregularly reflected, and renders visible that part of the eyebali. Those rays of the pencil which fall upon the cornea pass through it. The exterior rays fall upon the iris, by which they are irregularly reflected, and render it visible. The internal rays pass through the pupil, and are inci- dent upon the crystalline, which, being transparent, is also penetrated by them; they then pass through the vitreous humour, and finally reach the posterior surface of the inner part of the eye, where they penetrate the transparent retina, and are received by the black surface of the choroid, upon which they produce an illuminated spot. IMAGE ON THE RETINA. 567 The aqueous humour being more dense than the external air, and the surface of the cornea, which includes it, being convex, rays passing from the air into it will he rendered more convergent or less divergent. In like maimer, the anterior surface of the crystalline lens being convex, and that humour being more dense than the aqueous, a further convergent effect will be produced. Again, the posterior surface of the crystalline being convex towards the vitreous humour-, and this latter humour being less dense than the crystal- line, another convergent effect will take place. These rays passing suc- cessively through these three humours, are rendered at each surface more and more convergent. 895. Inverted Picture. — The eye, therefore, has the optical character and properties of a compound convergent lens, and will consequently form, at some point posterior to it, an optical image of any illuminated object which is presented before it. It is found that the refractive powers of the humours, and the form of their surfaces in eyes of ordinary visual power, are such that the principal focus of the organ is upon the retina at the posterior surface of the cavity, which is filled by the vitreous humour ; and consequently an inverted optical picture of any distant object placed before the eye will be projected upon this part of the retina. That this phenomenon is actually produced in the interior of the eye may be rendered experimentally manifest by taking the eyeball of an ox recently killed, and dissecting the posterior part, so as to lay bare the choroid. If the eye thus prepared be fixed in an aperture in a screen, and a candle be placed before it at a distance of eighteen or twenty inches, an inverted image of the candle will be seen through the choroid, as if it were produced upon ground glass or oiled paper. Fig. 424. The phenomenon can be still more manifestly shown by making an open- 568 ANIMAL PHYSICS. ing carefully at the upper part of the eyeball, so that the posterior part of the retina may be visible through the vitreous humour. In this case the image of any bright object, such as the window, to which the optic axis may be directed, will be seen depicted on the retina. The experiment mav be more easily performed, according to the method suggested by llagendie. by means of the eye of any albino animal, such as a white rabbit, in which the coats, from the absence of pigment, are transparent. Such an eye being dissected clean, and presented with its axis towards a window, a very distinct image of the window completely inverted will be seen depicted on the posterior semi-transparent wall of the organ. 896. Although the figures given above may suffice to show the relative forms and position of the parts composing the interior of the eyes, anatomical sections and views, taken under conditions of greater precision, are necessary to convey exact notions of the curious and complicated structure of the organ. In fig. 425 a vertical section of the left eye made in the median plane is given on a scale exactly twice the natural linear magnitude of the organ. The parts indicated are as follows : — Fig. 425. VERTICAL SECTION OF THE LEFT EVE (SappCy). 1. Optic nerve. 2, 3, 6, 7. Sclerotic. 4. External coat of optic nerve. 5. In- ternal coat of ditto. S. Motor muscles. 9 to 13. Cornea. 14. Canal of Fontana. 15, 16. Choroid. 17, 18. Ciliary processes. 19 to 21. Retina. 22. Central artery of retina. 23. Vitreous humours. 24. Hyaloid. 25. Zone of Zinn. 26, 27. Canal of Potit. 28. Crystalline lens. 29. Iris. 30. Pupil. 81. Posterior chamber. 32. Anterior chamber. 897. A segment of the iris, magnified four times in its linear dimensions, is shown in fig. 426. The arteries of the iris are represented in fig. 427. 898. Eye Achromatic and Aplanatic. — That the eye is sensibly achromatic is proved by the fact that the objects we EYE ACHROMATIC AND APLANATIC. 569 behold are not edged with coloured fringes, as is the case with all lenses which are not achromatic. But if, by any means, an object be seen out of focus, that is, so that its image shall fall either before or behind the retina, the achromatism ceases, and coloured fringes become apparent. The cases in which objects are thus seen out of focus will be presently indicated. It is also evident that the eye is aplanatic, or exempt from any sensible spherical aber- ration, since if it were not, the images on the retina, and consequently the per- ception of the objects pro- ducing them, would be more or less indistinct, which they are not. But if they are seen out of focus, as will presently appear, they become so. It appears, then, that the immediate cause of vision, and the immediate object of per- ception in the sensorium when we see, is the image thus produced by means of the refractive powers of the humours of the eye. Fig. 426. segment of mis magnified (Sappey). 1 to 5. Ciliary processes. 4. Bifurcated process. 6. Veins ramifying from sum- mits . 7. Festooned border of choroid. S. Veins of choroid. 10, 11. Borders of iris. 12. Radiating fibres of iris. 13. Circular fibres. 899. Other Analogies to an Optical Instrument. — Hot only does the iris play the part of the diaphragm provided in telescopes and microscopes to intercept the lateral rays and all stray light (being, however, more perfect than any ordinary diaphragm, inasmuch as it is capable of enlarging and contract- ing the opening according as circumstances require), but its posterior surface is coated with a black pigment, so that it cannot reflect the light which it intercepts. The posterior surface of the ciliary processes is covered with the same black pigment which coats the choroid, — a provision which has the same general effect in absorbing any rays of light which may be 570 ANIMAL PHYSICS. reflected within the eye, and preventing their being thrown again upon the retina, so as to confuse the image formed upon Fig. 427. arteries of iris (Arnold). A. Choroid. B. Iris. c. Ciliary ligament. 1. Posterior ciliary artery. 2. Anterior ditto. 3. Anastomosis of anterior with posterior. 4. Inner border of iris. i. Artery extending from outer to inner border. it. The black colour given to the inner surface of telescopes and microscopes is applied for a like purpose. 900. The Conditions of Perfect Vision are : — 1. The image on the retina must be perfectly distinct. 2. It must have sufficient magnitude. 3. It must be sufficiently illuminated. 4. It must continue on the retina for a sufficient length of time. 901. Distinctness of Image. — The image formed on the retina will be distinct or not, according as the pencils of rays are brought to an exact focus on the retina or not. If they be not brought to an exact focus on the retiua, their focus will be a point beyond the retina or within it. In either case the rays proceeding from any point of the object, instead of forming a corresponding point on the retina, will form a spot of greater or less magnitude, according to the distance of the focus of the CONDITIONS OF DISTINCT VISION. 571 pencil from tlie retina, and the assemblage of such luminous spots 'will form a confused picture of the object. This devia- tion of the foci of the pencils from the retina is caused by the refracting powers of the eye being either too feeble or too strong. If too feeble, the rays are intercepted by the retina before they are brought to a focus ; if too strong, they are brought to a focus before they arrive at the retina. 902. The objects of vision may be distributed into two classes, in relation to the refracting powers of the eye : 1st, those which are at so great a distance from the eye, that the pencils proceeding from them may be regarded as consisting of parallel rays : 2ndly, those which are so near that their rays have sensible divergence. It has been stated that the diameter of the pupil varies from 5 to J of an inch in magnitude, the variation depending upon a power of dilatation and contraction with which the iris is endued. Taking the diameter of the pupil at its greatest magnitude of a quarter of an inch, pencils proceeding from an object placed at the distance of three feet from the eye would have an extreme divergence amounting to about a third of a degree ; and if the pupil be in its most contracted state when its diameter is only one-eighth of an inch, then the divergence of the pencils proceeding from such an object would amount to about a sixth of a degree. It may therefore he concluded that pencils proceeding from all objects more distant from the eye than two or three feet may he regarded as consisting of parallel rays. The pencils of rays, therefore, proceeding from all such objects, will be made to converge to the principal focus of the eye. 903. Optical Centre. — Sir David Brewster concludes that the optical centre of the eye, that is to say, the point at which the axes of secondary pencils intersect the optic axis, is situate in the geometric centre of the eyeball, and consequently must be a little within the crystalline. If, therefore, round this centre we imagine a spherical surface described, whose radius is equal to the focal distance of the combination of the humours of the eye, the images of all objects more distant from the eye than two or three feet will be found on such a surface. Now, since the retina is spread over the surface of the choroid, and since the form of the eye is very nearly spherical, and its diameter but an inch, it follows that the retina is a concave spherical surface, whose centre coincides with the optical centre of the eye, and is at a distance from that centre of about half an inch. If the distance of the retina from this centre be exactly equal to the focal distance of the humours, then the foci of all pencils of parallel rays entering the eye will be formed upon it, and 572 ANIMAL PHYSICS. consequently it will receive distinct images of all objects whose distance from the eye exceeds two or three feet. But if the focal distance of the humours be less or greater, then, as already stated, the image on the retina will be indistinct. 904. Optical Remedies for Defects in the Refracting Powers of the Eye are supplied by the properties of conver- gent and divergent lenses. If the eye possess too littla con- vergent power, a convergent lens is placed before it, which, receiving the parallel pencils, renders them convergent when they enter the pupil, and this enables the eye to bring them to a focus on the retina, provided the power of the lens be equal to the deficient convergence of the eye. If, on the other hand, the convergent power of the eye be too great, so that the parallel rays are brought to a focus before arriving at the retina, a divergent lens is placed before the eye, by means of which parallel pencils are rendered divergent before they enter the pupil ; and the power of the lens is so adapted to the con- vergent power of the eye, that the rays shall be brought to a focus on the retina. The two opposite defects of vision here indicated are generally called, the one iveak-sightedness or far-sightedness, and the other near-sightedness. 905. Adaptation to Different Distances. — If it be admitted that the formation of a distinct picture at the posterior part of the eye be essential to distinct vision, and that the focus of the eye be regulated by the same principles as that of a convergent lens, it will necessarily follow that, supposing the eye to be so constituted as to have its principal focus on the retina, the foci of all pencils of divergent rays must necessarily be behind the retina. Now, since all objects at less distances from the eye than two feet transmit pencils sensibly divergent, the foci of all such pencils being behind the retina, the picture on the retina, and, consequently, the vision of the object, would be neces- sarily indistinct ; and the less the distance of the object from the eye, the greater would be the distance of the foci of the pencils behind the retina, and the more indistinct would be the vision. Nevertheless, it is found, in fact, that eyes which are capable of distinct vision at distances greater than two feet, are also capable of equally distinct vision at distances considerably within that limit. Thus, most eyes are capable of distinct vision at the distance of eight or ten inches, and many at even ADAPTATION TO DISTANCES. 573 less distances. It must therefore be inferred either that there is in the eye such a power of voluntary change, as is sufficient to vary its convergent power on the light transmitted through it, so as to bring forward to the retina the foci of rays diverging from points at eight or ten inches from it, or, that it is so con- stituted as to bring all pencils, which have a divergence less than those proceeding from objects at eight or ten inches distance, to an exact focus on the retina, without any change in its form or in the state of its humours. There is, perhaps, no point in physical science upon which more diversity of opinion has prevailed than this. Some eminent physiologists, among whom may be named De la Hire, Haller, Magendie, Simonoff, and Treviranus, have absolutely denied, as a matter of fact, that the eye does undergo any change of form or state in looking at distant and near objects ; and the last-mentioned of these philosophers has professed to demonstrate that such a constitution of the humours is possible as would cause all the pencils, whose divergence varies within the supposed limits, to come to a focus on the retina. Not only, however, has the validity of the reasoning by which Tre- viranus supports his hypothesis been called in question, but it has been demonstrated, as a matter of fact, that the state of an eye which sees distinctly objects at eight inches or less dis- tance, is different from the state of the same eye when it sees distinctly objects at distances exceeding two or three feet. This has been established by various experiments. 906. If we close one eye, and place two pins in the direc- tion of the axis of the other, one at eight inches and the other at twenty-four inches distance, so as not actually to intercept each other, it will be found that the eye cannot see distinctly both pins at the same time, but that by a voluntary act it can render the vision of one or the other distinct. If by this voluntary effort the more distant pin is seen distinctly, the nearer pin will be indistinct, and if, on the other hand, the nearer pin be seen distinctly, the more distant pin will be indistinct. It is clear, therefore, that the convergent power of the eye is varied, so that in the one case it brings rays which are sensibly parallel to a focus on the retina, while the focus of rays sensibly divergent is behind the retina ; and that in the other case, the latter rays are brought to a focus upon the retina, while the focus of the former is in front of it. * * See Handbook Nat. Phil., Optics, § 335, et seq. 574 ANIMAL PHYSICS. 907. The Visual Magnitude, or apparent magnitude, i> ’he angle formed by the visual lines drawn to the extreme limits of the object. Thus, visual differs from real magnitude, the latter being measured by lines, while the former is measured by angles. The visual magnitude of an object varies with the distances of the observer from it, inasmuch as every increase of that dis- tance causes a proportionate decrease of the visual angles, and vice versa. Objects having very different real magnitudes may, therefore, have the same visual magnitude. This will occur if their distances from the eye be in the exact ratio of their linear dimensions. The sun and moon present a remarkable example of this. The minor limit of the visual angle capable of being distinctly perceived varies with the colour of the object and the intensity of its illumination. Plateau affirms that a white disc, the sun shining fully upon it, will be visible when seen under a visual angle of twelve seconds, or the one-fifth part of a minute. The disc would subtend this angle at the eye if placed at a distance equal to 17250 times its diameter. He says also that if the disc were red, it would be distinctly seen until its apparent magnitude were reduced to twenty- three seconds ; and that if it were blue, the limit would be twenty- six seconds ; but that, if instead of being illuminated by the direct solar light, it were illuminated by the light of day reflected from the clouds, these limiting angles would be half as large again. 908. The Minuteness of the Ocular Pictures which pro- duce distinct vision is truly astonishing. If we look at the full moon on a clear night, we perceive with consider- able distinctness, by the naked eye, the lineaments of light and shade which characterise its disc. Now let us consider only for a moment, what are the dimensions of the picture of the moon formed on the retina, from which alone we derive this distinct perception. The disc of the moon subtends a visual angle of half a degree, and consequently, according to what has been explained, the diameter of its picture on the retina will be Jjjth part of an inch, and the entire superficial magnitude of the image from which we derive this distinct perception is less than the sjjtnth of a square inch ; yet within this minute space we are able to distinguish a multiplicity of still more minute details. We perceive, for example, forms of light and shade, whose linear dimensions do not exceed one tenth part of the apparent diameter of the moon, and which therefore occupy upon the retina a space whose area does not amount to the 1* 0 a square inch. SUFFICIENCY OF ILLUMINATION. 575 909. To take another example, the figure of a man 70 inches high, seen at a distance of 40 feet, produces an image upon the retina the height of which is about one fourteenth part of an inch. The face of such an image is included in a circle whose diameter is about one-twelfth of the height, and therefore occupies on the retina a circle whose diameter is about the ffijth part of an inch ; nevertheless, within this circle, the eyes, nose, and linea- ments are distinctly seen. The diameter of the eye is about one-twelfth of that of the face, and therefore, though distinctly seen, does not occupy upon the retina a space exceeding the jnsWi^1 of a square inch. If the retina be the canvas on which this exquisite miniature is delineated, how infinitely delicate must be its stru cture to receive and transmit details so minute with such marvellous precision ; and if, according to the opinion of some, the perception of these details be obtained by the retina feeling the image formed upon the choroid, how exquisitely sensitive must be its touch ! 910. — Sufficiency of Illumination is as necessary to distinct vision as a well-defined ocular picture. Thus it is possible to conceive a picture on the retina, so extremely faint as to be insufficient to produce sensation, or, on the other hand, so intensely brilliant as to dazzle the eye, to destroy the distinct- ness of sense, and to produce pain. When we direct the eye to the sun, near the meridian, in an unclouded sky, we have no distinct perception of his disc, because the splendour is so great as to overpower the sense of vision just as sounds are sometimes so intense as to be deafening. That it is the intense splendour alone which prevents a distinct perception of the solar disc in this case, is rendered manifest by the fact that if a portion of the solar rays be intercepted by a coloured glass, or by a thin cloud, a distinct image of the sun will be seen. When we direct the eye to the firmament on a clear night, there are innumerable stars which transmit light to the eye, and which therefore must produce some image on the retina, but of which we are altogether insensible, owing to the faintness of the illumination. That the light, however, does enter the eye and arrive at the retina is proved by the fact, that if a telescope be directed to the stars in question, so as to collect a greater quantity of their light upon the retina, they will become visible. The eye possesses a certain limited power of accommodating itself to various degrees of illumination. Circumstances which are familiar to every one render the exercise of this power evident. If a person after remaining a certain time in a dark room, pass sud- denly into another room strongly illuminated, the eye suffers instantly a degree of inconvenience, and even pain, which causes the eyelids to close ; and it is not until after the lapse of a certain time that they can be opened without inconvenience. Effects, the reverse of these are observed when a person passes from a strongly illuminated room into one comparatively dark, or into the open air at night. For a certain time he sees nothing, because the contraction of the pupil, which was adapted to the strong light to which it had pro- 57G ANIMAL PHYSICS. viously been exposed, admits so little light to the retina that no sensation is produced. The pupil, however, soon dilates, and, admitting more light, objects are perceived which were before invisible.* 911. Foramen Centrale and Limbus Luteus, or Yellow Spot — That part of the retina which immediately surrounds the point of it to which the optic axis is directed, Is attended with several circumstances which ought not to be passed over here. The point where the optic axis meets the retina is the centre of a circular yellow spot, called the limbus lutem, the radius of which. is about the sixteenth of an inch. In its centre, and therefore at the extremity of the optic axis, is what has the appearance of a minute hole, and has accord- ingly been called from its discoverer, the foramen centrale of Sommering. It is, however, considered by anatomists that this is not a real opening between the vitreous humour and the choroid, inasmuch as a layer of vascular matter covers it, the opening being only in the medullary substance of the retina at that particular point. The distance between the foramen centrale and the centre of the embouchure of the optic nerve is about the tenth of an inch ; and since the radius of the yellow spot is the sixteenth of an inch, it follows that the edge of the yellow spot is about the twenty-seventh of an inch from the centre of the optic nerve. Taking the radius of the concave spherical surface formed by the retina to be half an inch, 1° upon it will correspond to the 115th part of an inch ; and, consequently, the angle subtended by the semidiameter of the yellow spot at the optical centre of the eye will be 7°, and the angle subtended by the distance between the foramen centrale and the centre of the embouchure of the optic nerve will be lll°. The sensitiveness of the retina is not the same at all points. If we direct the optic axis to any point upon a distant object, a certain extent of that object surrounding the point to which the optic axis is directed will be visible, but not with a uniform vividness and distinctness. The point to which the axis is directed will be seen with greatest distinctness, and the sur- rounding points will be perceived with less and less distinctness, as they are more distant from this central point, until they altogether disappear. * Handbook Nat. Phil., Optics, § 362. FIELD OF VISION. 577 912. Field of Vision. — The extreme mobility of the eye, and the subtle and unconscious action of the will upon it, render it extremely difficult to keep the axis fixed upon a certain point while the visual perception of the surrounding points is attempted to he observed. The moment we desire to ascertain to what visual distance on any side of the central point our perception extends, the optic axis, with the rapidity of thought, directs itself to such points. Nevertheless, by much practice, such self-control can be acquired as will enable an observer to ascertain with some degree of approximation the extent of the field of vision, by which term is expressed the circular space surrounding the point to which the optic axis is directed, which includes all the objects which can be per- ceived by the eye at the same instant. The circle of the retina surrounding the foramen centrale, which corresponds to this field of vision, includes the entire extent of that membrane which is available for the sense of sight : for, although the range of the eye is really much greater, that extension of its sphere of perception is due to the mobility of the eyeball, by which, as already explained, the optic axis has a play, measured horizontally and vertically, through a con- siderable angular space. To determine, by immediate observation, the extent of the field of vision when the optic axis is fixed, let a number of red wafers be attached at short intervals to the circumference of a circle having a whitish ground two feet in diameter, and let a single wafer be attached to its centre. Let the card or pasteboard upon which the wafers are attached be fixed to a vertical wall, so that the central wafer shall be at the level of the eye of the observer standing with his face to the wall. If the observer, closing one eye, the left, for example, stand so that aline drawn from the other eye to the central wafer shall be perpendicular to the plane of the circle, and so that his distance from the wall shall be ten or twelve feet, he will see the entire circle of wafers when the optic axis of his eye is directed to the central wafer. If then he gradually approach the circle, still keeping the optic axis directed upon the central wafer, the circumferential wafers will continue to be visible, but will be gradually less and less distinct. When he approaches to the distance of five feet from the central wafer, a remark- able effect will ensue. Those circumferential wafers which are at and near the right-hand extremity of the horizontal diameter of the circle will sud- denly cease to be visible, and a gap will appear in the circle on the right side, extending over a fifth or sixth part of the entire circumference. If the observer now approach still nearer to the circle, keeping the optic axis still directed to the central wafer, the right-hand wafers will continue to be invisible, until he comes within something less than three feet of the central wafer, when they will s'uddeuly reappear. But upon approaching still nearer, all the circumferential wafers will vanish, the central wafer alone being visible. p p 578 ANIMAL PHYSICS. To explain these phenomena, it must be observed that at the dl-.tance of ten feet the radius of the circle is seen under a visual angle of 5 '7°, 'w hich corresponds to the 20th of an inch upon the retina. The retinal image of the circle of wafers will therefore be a circle having a radius of the 20th of an inch described round the foramen centrale ; it will therefore fall within the yellow spot ; and, as in this position the observer see* with considerable distinctness the circumferential wafers, and with perfect distinctness the central wafer, it follows that the sensibility of the retina corresponding to the yellow spot is within this limit suffi- cient for distinct vision, the central point, however, being the most sensitive and producing the most distinct perception. As the observer approaches the circle, the diameter of the image on the retina increases in the same proportion, very nearly as the distance of the eye from the centre of the circle diminishes. At the distance therefore of five feet, the radius of the retinal image is increased to the tenth of an inch, and that part of it which is on the side of the nose consequently passes across the embouchure of the optic nerve ; and as this corresponds to that part of the circle which in this position of the observer becomes invisible, it follows that that part of the retina which corresponds with the embouchure of the optic nerve is absolutely insensible. That this is the true explanation of the phenomenon is proved by the fact, that when the observer approaches within less than three feet of the central wafer, the circumferential wafers which were before in- visible suddenly reappear. In that case the image of the circle on the retina is so enlarged that its circumference includes within it the entire embouchure of the optic nerve, so that the wafers which at five feet distance projected tlieir images upon the embouchure of the optic nerve, now project them on that part of the retina which lies outside the nerve. Since the nerves are the only conduits between the organs of sense and the brain, it must have appeared somewhat inexpli- cable that the foramen centrale, the only point of the retina where practical anatomists were unable to discover the presence of nervotis matter, should not only possess visual sensibility, but should be endowed with that power in a higher degree than any other part of the retina. It could not, therefore, be matter of surprise that the result of their observations was received with much doubt, more especially as the nervous fibres are highly microscopic, and might be regarded as pro- bably more and more minute, as their sensibility is more exalted. Careful research in recent years, however, has shown that certain elements of the retina exist in the foramen centrale, or, as it is now called by some anatomists, the fovea centralis. The minuteness, however, of these elements is such, that it is not surprising that they should have escaped the observation of microscopist anatomists.* * Quain’s Anatomy, Sixth Edit. vol. iii. p. 24. LIMITS OF CIRCLE OF VISION. 579 913. The Limits of Field of Distinct Vision, while the optic axis has a fixed direction, has not been satisfactorily determined. I find by my own observations that objects com- prised within a circle of a foot radius, described round the point to which the optic axis is directed, are visible with suffi- cient distinctness for all the purposes of vision when the eye is placed at the distance of about six feet from the circle. This would correspond to a visual circle of nearly 20° radius ; so that if the optic centre of the eye bo supposed to be the apex of a cone whose angle is about 40°, all objects within that cone will be visible at the same moment with suffi- cient distinctness when the optic axis has the direction of the axis of the cone. When the eye approaches nearer to such a circle, the objects comprised within it, with the exception of those rendered in- visible by the insensibility of the embouchure of the optic nerve, are still seen, but the perception of them is indistinct and unsatisfactory. It is probable, however, that these limits of distinct vision measured from the optic axis as a centre, may be different in different eyes. Valentin gives the narrow limit of 3° round the optic axis, as the range of distinct vision. This nrust certainly be an error. He does not state on what authority nor on what kind of experiment or observation his conclusion is based. A radius of 20° corresponds to about the sixth of an inch upon the retina, and if the conclusion derived from my own observations be correct, it will follow that the portion of the retina available for distinct vision will be a circle described round the foramen centrale as a centre, with a radius of about the sixth of an inch. 914. Attention Necessary to Perception. — In enumerating the conditions necessary to ensure the distinct perception of a visible object, we have in what precedes included those only which are strictly optical ; there is, however, a mental condi- tion not less necessary to perception than the optical conditions already mentioned. The mind has the power by an act of the will to direct its attention Avith more or less exclusiveness to certain perceptions or ideas, whether proceeding directly from external objects, or evoked by memory or imagination, in pre- ference to others ; and, in such cases, although all the condi- tions of distinct vision above enumerated may be fulfilled, no distinct perception, or no perception at all, may be produced, i* r 2 580 ANIMAL PHYSICS. owing to the attention of the mind being diverted to other objects. This is not peculiar to sight, but common to all sensible impressions. When engrossed in thought upon any subject of deep interest, we often have our eyes open and fixed upon external objects, from which the retina receives impressions fulfilling all the conditions of distinct vision, yet we see nothing. Physiologists explain this by stating that the fibres of the optic nerve, although transmitting to the sensorium the action produced upon the retina, fail to produce a percep- tion there because the sensorium is then preoccupied by other thoughts and perceptions. Although this, instead of explain- ing the phenomenon, is little more than a statement of it, it is the only solution offered of a question which lies upon the con- fines of physiology and psychology. “ But by tins faculty of attention, we also analyse what the field of vision presents. The mind does not perceive all the objects presented by the field of vision at the same time with equal acuteness, but directs itself first to one and then to another. The sensation becomes more intense according as the particular object is at the time the principal subject of mental contemplation. Any compound mathematical figure produces a different impression, according as the attention is directed exclusively to one or the other part of it. Thus, in fig. 428, we may in succession have a vivid perception of the whole, or of distinct parts only ; of the six triangles near the outer circle, of the hexagon in the middle, or of the three large triangles. The more numerous and varied the parts of which a figure is composed, the more scope does it afford for the play of the attention. Hence it is that architec- tural ornaments have an enlivening effect on the sense of vision, since they afford constantly fresh subject for the action of the mind.”* Pig. 428. 915. Binocular Vision. — The optical phenomena which we have hitherto considered and explained, are such as would be produced in an observer having a single eye, and, as distin- guished from certain others, may be denominated monocular ; the peculiar phenomena depending on the simultaneous vision with two eyes being called binocular. This being, however, a point which belongs more properly to optics than to physiology, and one which cannot be satisfac- M tiller's "Physiology,” vol. ii., p. 1179. COLOUli-BLlNDNESS. 581 torily explained with the brevity necessary for our present pur- pose, we must refer the reader for information upon it to our Handbook of Natural Philosophy, Optics, § 404 et seq. 916. Perception of Colours. — The immediate impressions re- ceived from the sense of sight are those of light and colour. The impressions of distance, magnitude, form, and motion, are the mixed results of the sense of sight and the experience of. touch. Even the power of distinguishing colours is not obtained immediately by vision, without some cultivation of this sense. The unpractised eye of the new-born infant obtains only a general perception of light ; and it is certain that the power of distinguishing colours is only acquired after the organ has been more or less exercised by the varied impressions produced by different lights upon it. It would not be easy to obtain a summary demonstration of this proposition, from the experience of infancy, but sufficient evidence to establish it is supplied by the cases in which sight has been suddenly restored to adults blind from their birth. In these cases, the first impression produced by vision is, that the objects seen are in immediate contact with the eye. It is not until the hand is stretched forth, so as to ascertain the absence of the objects seen from the space before the eye, that this optical illusion is dissipated. The eye which has recently gained the power of vision cannot at first distinguish one colour from another, and it is not until time has been given for experience that either colour or outline is perceived. 91 T. Colour-Blindness. — Besides that imperfection incident to the organs of sight, arising from the excess or deficiency of their refractive powers, there is another class, which appears to depend upon the quality of the humours, through which the light proceeding from visible objects passes before attaining the retina. If these humours be not absohitely transparent and colourless, the image on the retina, though it may corre- spond in form and outline with the object, will not correspond in colour, supposing the nerves and sensorium to be in a healthy state ; for if the humours be not colourless, some constituents of the light proceeding from the object will bo intercepted before reaching the retina, and the picture on the retina will accordingly be deprived of the colours thus inter- cepted. If, for example, the humours of the eye were so constituted as to intercept all the red and orange rays of white light, white paper, or any other white object, such as the sun, 582 ANIMAL PHYSICS. for example, would appear of a bluish-green colour ; and if, on the other hand, the humours were so constituted as to intercept the blues and violets of white light, all white objects would appear to have a reddish hue. Such defects in the humours of the eye are fortunately rare, but not unprecedented. Sir David Brewster, who lias curiously examined and collected together cases of this kind, gives the following examples of these defects : — A singular affection of the retina, in reference to colour, is shown in the inability of some eyes to distinguish certain colours of the spectrum. The persons who experience this defect have their eyes generally in a sound state, and are capable of performing all the most delicate functions of vision. Harris, a shoemaker at Allonby, was unable from his infancy to distinguish the cherries of a cherry-tree from its leaves, in so far as colour was concerned. Two of his brothers were equally defective in this respect, and always mistook orange for grass-green, and light green for yellow. Harris himself could only distinguish black and white. Mr. Scott, who describes his own case in the “Philosophical Transactions,” mistook pink for a pale blue, and a full red for a full green. All kinds of yellows and blues, except sky-blue, he could discern with great nicety. His father, his maternal uncle, one of his sisters, and her two sons, had all the same defect. A tailor at Plymouth, whose case is described by Mr. Harvey, regarded the solar spectrum as consisting only of yellow and light blue ; and he could distinguish with certainty only yellow, white, and green. He regarded indigo and Prussian blue as black. Mr. It. Tucker described the colours of the spectrum as follows : — Red mistaken for ....... brown. Orange „ . . . . . . . green. Yellow sometimes ...... orange. Green . . . . . . . . orange. Blue ,, ....... pink. Indigo ,, ....... purple. Violet „ ....... purple. A gentleman in the prime of life, whose case I had occasion to examine, saw only two colours in the spectrum, viz., yellow and blue. When the middle of the red space was absorbed by a blue glass, he saw the black space with what he called the yellow on each side of it. This defect in the perception of colour was experienced by the late Mr. Dugald Stewart, who could not perceive any difference in the colour of the scarlet fruit of the Siberian crab, and that of its leaves. Dr. Dalton was unable to distinguish blue from pink by daylight ; and in the solar spectrum the red was scarcely visible, the rest of it appearing to consist of two colours. Mr. Troughton had the same defect, and was capable of fully appreciating only blue and yellow colours ; and when he named colours, the names of blue and yellow corresponded to the more and less refrangible rays : all those which belong to the former exciting the sensation of blueness, and those which belong to the latter the sensation of yellowness. 918. Case of Dr. Dalton. — In almost all these cases the different prismatic colours had the power of exciting the sensation of light, and giving a distinct vision of objects, excepting in the case of Dr. Dalton, who was said to be scarcely able to see the red extremity of the spectrum. He endeavoured to explain this peculiarity of vision, by supposing that in his VISION OF ANIMALS. 583 own case the vitreous humour was blue, and therefore absorbed a great portion of the red and other least refrangible rays. That this opinion was erroneous, however, was proved by the post mortem dissection of the eyes of that eminent person, by which it appeared that the vitreous humour was perfectly transparent and colourless. Sir John Herschel attributes the defect of Dr. Dalton’s vision, and other defects of the same class, to a morbid state of the sensorium, by which it is rendered incapable of appreciating exactly those differences between rays, on which their colour depends. * 919. The Visual Organs of Inferior Animals are subject to considerable variety of form, structure, and position, but may be grouped in three classes : — 1st. Eyes formed with transparent media, by which the visual rays are refracted to foci, so as to form images. 2nd. Compound eyes, also furnished with transparent media receiving light from different objects or from different parts of the same object. 3rd. Simple eyes, or eye-dots, which appear to possess no other visual faculty except that of distinguishing light from darkness, as if a piece of hom or ground glass were placed before the eyes of the superior animals. 920. Vertebrata. — The optical structure of the eyes of this division of the animal kingdom, while it is more or less analogous to that of the human organ, is subject, nevertheless, to great variety in its details. The eyelids are in some cases altogether absent, the skin covering the eye, — as, for example, in the proteida among the amphibia, and the pipa among fishes- The skin sometimes forms a sort of circular zone with a central opening, as in the chameleon. Besides the eyelids analogous to those in the human organ, there is in some animals a third, called the membrana nictitans. This is fully developed in birds and reptiles, and exists in a less perfect state in certain genera of the shark family among fishes. In birds this is a semi-transparent membrane, which can be drawn from the inner comer over the entire surface of the eye by means of an appropriate muscular apparatus. 921. The Lachrymal Apparatus is absent in cetacea, am- phibia, and fishes. The sclerotic in some animals has a tendency to become cartilaginous, and even bony. In birds, tortoises, and lizards, this membrane on the borders of the cornea is supplied with a ring of bony plates, the edges of which either overlap one another or are placed in close apposi- tion. The sclerotic of fishes consists generally of two large coats of cartilage. * Handbook Nat. Phil., Optics, § 443. 584 ANIMAL PHYSICS. 922. The Choroid iu animals is divisible into two lamina?, tlie external and internal. The inner surface of the choroid i - covered with the membrana pigmenti, consisting of flattened cells, usually hexagonal, containing granules of pigment. In albinos the cells are destitute of pigment. In some animals tin- membrane is deficient at certain parts of the eye, which have then either a metallic lustre or are white. This part of the internal surface of the eye is called the tapetum. The metallic lustre, or iridescent colour, is explained by the interference of the light reflected from it, and is a phenomenon similar to that produced by mother-of-pearl, and other bodies of laminated structure. The tapetum varies in colour in different animals. In the ox it has a greenish metallic hue ; in the cat a golden yellow ; in the horse a silvery blue, and so on. In carnivorous animals it is perfectly white, and occupies a space accurately triangular at the posterior surface of the eye. 923. The Iris, in most animals, is contractile, as in man ; but in bony fishes is not perceptibly so. 924. The Pupil is sometimes round, as in the human eye ; sometimes elongated transversely, as in ruminants ; sometimes vertically, as in the cat family and the crocodile ; sometimes angular, as in the brown or fire toad. 925. The Pecten, or IVTarsupium. is a pyramidal plicated process from the choroid coat. Passing through an opening in the retina in the bottom of the eye into the middle of the vitreous humour, it follows the direction of the borders of the crystalline.' It is characteristic of the class of birds. 926 The Microscopic Structure of the Retina of Animals has been recently investigated by Treviranus, Gottsche, and others. This membrane is found to consist of three principal layers : an external, pnlpy, and granulated ; a middle, fibrous ; and an internal layer consisting of erect cylinders, being a con- tinuation of the middle or fibrous layer. On entering the eye, the optic nerve divides into cylindrical nervous fibres, which radiate over the middle fibrous layer. Each of these fibres at a certain part of its course turns at right angles to its direction, and terminates in a papilla, so that the combination of all these fibres forms a papillary surface over the entire retinal coat of the eye. A section of tlie retina of the hooded crow (eornis comix) is shown in fig. 429, where a is the internal lamina of the choroid ; b, the first layer, is STRUCTURE OF THE EYES OF ANIMALS. 585 a cellular tissue in which the fibres of the retina radiate ; c, these fibres turned at right angles to their course, as already described ; d, the second layer, of cellular tissue ; e, the larger nervous fibre ; /, the third layer, perforated by the papillae g, in which the nervous c fibres terminate. The retinal fibres here described vary in their magnitude in different species. The following are the diameters of the species indicated in millionths of an inch : — Hedgehog fibres ... 40 Rabbit papillae . . . 132 Birds’ papillae . . . 80 to 160 Frogs’ fibres .... 176 Ditto papillae .... 264 Fig. 429. 92 <, Iii animals wkicli seek section of the retina of a their prey by night, the eyes are hooded crow (Treviranus). generally larger than those which obtain their nourishment by clay. The structure of the pupil in the former differs from that of the human eye, which in contracting or expanding still retains its circular form. It would appear that the degree of play necessary for the pupil of nocturnal animals cannot be obtained by a system of radiating muscles. The pupil, there- fore, in these species in contracting assumes an elliptical form, which under the extreme action of the muscles is reduced to a narrow cleft crossing the iris like a button-hole. It is by this expedient that the retina is defended from the injuriously intense action of light during the day, while the fully expanded circular pupil admits sufficient light at night to produce a sensible effect upon it. 928. It will be remembered that the refracting power of the humours of the eye depends, not alone upon their own density and form, but also upon the density of the sur- rounding medium. According as the latter is increased or diminished, the refracting power of the organ of vision must necessarily also be increased or diminished. It will therefore be apparent that the refracting power of the eyes of animals which live in water recpiires to be greater than that of animals living in the atmosphere, in the same proportion as the refracting power of water is greater than that of air. In accordance with this, it is found that the crystalline lens in aquatic animals is much more convex than in land animals, and consequently the refracting power of the eye is proportionally greater. 586 ANIMAL PHYSICS. 929. In most mammifers tlie eyes are so mounted in sockets that the direction of the optic axis in their normal position is more or less lateral, so that they can never be directed both to the same object, being always divergent. One of the distin- guishing characteristics of human vision is the direction of the optic axes, which, in their normal position are parallel to each other, horizontal, and at right angles to the axis of the body. No part of the economy so obviously indicates the erect posi- tion as this. It is only when man stands erect that he can look before him without any extraordinary effort. As we descend in the scale of organisation we find that, as intelligence is lowered, the eyes are placed more and more laterally, so that, in several species, the sphere of vision of each eye is wholly different from that of the other, and it would seem that, in some cases, the animal is altogether incapable of looking before it. 930. Birds have the sense of sight highly developed. The eyes are proportionally larger than in mammifers, and supplied with certain supplementary arrangements. The retina is thick, and connected with the crystalline by means of the pecten (925). The pupil is always round, and the cornea large. That this class is endowed with extraordinary powers of adaptation to vision, at all distances, is certain ; but on what provision this power depends has not been ascertained. The sclerotic is strengthened in front by the zone or hoop of bony plates, already mentioned, surrounding the cornea. The eye- lids, as already stated, are triple — two being horizontal, and opening with a vertical motion as in mammifers, and the third vertical, opening with a horizontal motion. The latter is placed at the internal angle of the eye, and is capable of cover- ing the entire surface of the organ. It is semi-transparent. Of the two horizontal eyelids, the inferior, contrary to the dispo- sition in mammifers, is the larger and more mobile. 931. The optical structure of the eye of different species of birds is adapted in the most admirable manner to their varied wants. Thus, in the case of birds of more limited flight, the mechanism is not furnished with any special provision to enable them to see distinctly at a greater distance than their habits render necessary. In the case, however, of those which rise into higher regions of the air, and whose prey is confined to the surface of the ground, nature has provided a special apparatus by which, at will, the bird can render its eye telescopic, so as to distinguish perfectly EYES OF BIRDS AND REPTILES. 587 its minute prey on the ground from heights so great, that the bird itself, though voluminous, is scarcely distinguishable to human vision. Nevertheless, so distinct and sure is its vision that it drops upon its prey with the most unerring precision. With such species the crystalline is less dense and convex than with birds of more limited flight, and the power of the eye to confer upon itself this long sight, is supposed to depend upon the motor muscles, which, acting upon the bony hoop of the sclerotic, compress the humours with which the organ is filled, or, on the contrary, by their relaxation, relax them. In the one case, the cornea is rendered more, and, in the other case, less convex, so that the focal length of the eye is increased or diminished at will, within veiy wide limits. The power of expanding the pupil by admitting a larger pencil of rays to the crystalline, and probably that of advancing the pecten already mentioned, are combined with this in pro- ducing the desired effect. 932. Reptiles have often, like birds, three eyelids, though sometimes, as in the case of serpents, there are none. The eye- ball is then, as in fishes, only covered by a semi-transparent conjunctiva. With most of them the lachrymal glands are rudimentary. The form of the crystalline varies with the habits of the animal, being more convex with aquatic reptiles, than with terrestrial. With some reptiles the pecten of birds is found in a rudimentary state. Some inferior reptiles, such as the proteida and csecilians, which five in the water of obscure caves, or which hollow for themselves holes in dark and humid places, have rudimentary eyes, consisting of a capsule filled with a transparent liquid, lined internally by a sort of retina, and covered with a pigment on the external surface, from which only the part of the capsule directed to tho surface is free. The eyes are hidden under the integuments in the middle of a subcutaneous cellular tissue. These animals can only have very imperfect sight. 933. The Eyes of Fishes are large and very slightly move- able, having neither eyelids nor a lachrymal apparatus. Tho skin passes over them, and is sufficiently transparent to allow light to pass through it. The cornea has but little curvature, the pupil being large, with little contractile power, and the crystalline spherical. A remarkable anomaly in the position of the eye is presented 588 ANIMAL PHYSICS. in the case of soles and flat-fish, these organs being placed not as usual, one at each side of the head, but both at the same side. This however, is quite in accordance with a similar want of symmetry in other parts of the body of these species. 934. Annulata. — Insects and Crustacea have compound eyes, consisting of an agglomeration of a vast number of conical tubes, radiating from a centre, and terminating at the external part of the organ so as to form a spherical surface of greater or lesser extent. These cones are terminated at their external surface by little comese of polygonal and generally hexagonal forms. Each of them includes in its interior a humour analo- gous to the vitreous humour of the human eye, and receives a nervous filament at the internal extremity. Their inside surface is coated with dark pigment. Each eye, although its diameter does not usually exceed a minute fraction of an inch, is composed of a vast number of these polygonal tubes, sometimes so many as from ten to twenty thousand. The cornea itself, which covers the external ends of these tubes is coated at its edges with the same opaque pigment which lines the sides of the tube itself, the middle part only being transparent. d Fig. 430. SECTION OF THE EYE OF THE COCKCHAFER (Strauss Durckheim). 035. To illustrate the structure of these compound eyes, we give in fig. 420. after Strauss Durckheim, a section of the compound eye of a species of beetle (melolontlia vulgaris), where the facets of the cornea are mwrrm lUUlN-Jx** Fig. 431. SEGMENT OF THE SAMI: EYE (MUller.) VISION OF ANNULATA AND ARACHN1DA. 589 shown at a. The parts supposed hy Strauss to be enlarged extremities of the nervous fibres, but shown by Muller to be transparent cones surrounded with pigment, appear at b; the fibres of the optic nerve at c; and the trunk of the nerve at d . A section of the same, more highly magnified, is given after Muller in fig. 431, where the prismatic segments or facets of the cornea are shown at a; the transparent conical crystalline bodies at b; and the fibres of the optic nerve at c. In fig. 432, the membrane forming the cornea of the compound eye of a common house-fly is shown, magnified 100 times its lineal dimensions. 936. According to the observations of various eminent natu- ralists, such as Swammerdam, Leeuwenhoeck, Barter, Reaumur, Lyonnet, Paget, Muller, Strauss, Duges, Kirby, and others, the following are the number of eyes in certain species : — NUMBER OP EVES. The ant and xenos . i .50 1 The Goat moth . . 11300 The Sphynx . 1300 [ The dragonfly . 12544 The common fly . . 4000 1 The butterfly . 17355 The silkworm . . 6236 The mordella . 25088 The cockchafer . SS20 | 937. Arachnida. — The eyes of this class are constructed Fig. 432. upon the same general principles as those of the vertebrata but consisting of parts having very different forms. 590 ANIMAL PHYSICS. The organ may be illustrated generally by the eye of the scorpion, shown in fig. 433. Fig. 433. SECTION OF THE EYE OF A SCORPION magnified (Muller). In many insects, and in some Crustacea, compound and simple eyes coexist. The simple eyes, three or four in number, are then generally placed at the summit of the head, between the two compound eyes. It is probable that the simple eyes are used only for the vision of near objects, and especially to distinguish food, while the compound eyes direct the animal in its movements. Behind the cornea a is a spherical lens, b representing the crystalline, and behind tbi3 the vitreous hum our d, having a lenticular form surrounded by the choroid c, and resting on the retina e, having a cup-sliaped form, and being the continuation of the optic nerve/. 938. The Molluscous Cephalopods have eyes analogous to those of the superior animals. The poulp and the cuttle-fish have two large eyes lodged at the sides of the head, composed of sclerotica, choroid, retina, cornea, vitreous humour, and crystalline lens, with rudiments, occasionally, of eyelids. 939. The Gasteropods (snails, Ac.) have eyes supported upou salient peduncles. They are less perfect than the pre- ceding, consisting only of a vesicle coated with pigment, filled with vitreous humour, and having a transparent point in front. Some molluscous acephala, and probably also some radiata, present upon certain points of the body vesicles coated ■with pigment, which are sometimes called eye-dots, and which endow them apparently with the power of distinguishing light from darkness, but not properly with any powers of vision. HEARING. 591 CHAPTER XYI. HEARING. 940. The eye is so evidently adapted to the properties of light, and the purpose which each of its parts is destined to fulfil can be so clearly demonstrated, that a like conformity might naturally be expected between the form and structure of the ear and the physical properties of sound. Nevertheless, with one or two exceptions, the peculiar and complicated form of the organ of hearing has not been hitherto shown by any satisfactory or conclusive reasoning to be related to the prin- ciples of acoustics. 941. The Ear consists of three distinct compartments, dif- fering extremely from each other in their form. They are named by anatomists according to then’ order — proceeding from without inwards — the external, middle, and internal ear. 942. The External Ear. — The part of this division of the organ visible on the outside of the skull, behind the joint of the lower jaw, is called the pinna or auricle. The several parts marked in the figure by the numbers 1, 2, 3, &c., are distinguished by specific names in anatomy. With the exception, however, of the cavity 7, called the concha, none of these parts can be con- sidered as having any important acoustic properties. The depression, 2, called the fossa of the helix, and the surrounding car- tilage, 1, called the helix, may possibly have some slight effect in reflecting the rays of sound towards the concha, 7, and thence into the interior of the ear. If such, how- ever, were the purpose, it would be much more effectually answered by giving to this part of the organ a form more closely re- sembling that of the wide end of a trumpet. Fig. 434. As the external ear is actually constructed, the only part which answers this purpose is the concha. 592 ANIMAL PHYSICS. 943. External Meatus. — Proceeding inwards from the concha, the remainder of the external ear is a tube something more than an inch long, the diameter of which becomes rapidly smaller ; its calibre is least about the middle of its length, being slightly augmented between that point and its connection with the middle ear. Its section is everywhere elliptical, bnt in the external half the greater diameter of the ellipse is vertical, and in the internal, horizontal. This tube does not proceed straight onwards, but is twisted so that the distance from the concha to the point where it enters the middle ear is less than the total length of the tube. The external part of the tube is cartilaginous like the external ear, but its internal part is bony ; the bony surface, however, being lined by a pro- longation of the skin of the auricle. 944. Membrane of Tympanum. — Tlie internal extremity of this tube is inserted into an opening leading into the middle ear, Fig.435. which is inclined to the axis of the tube .at an angle of about 45°. Over this opening, which is slightly oval, an STRUCTURE OF THE EAR. 593 elastic membrane, called the membrane of the tympanum, is tightly stretched, like parchment on the head of a drum. In fig. 435 the several parts of the ear are shown divested of the surrounding bony matter ; and to render their arrangement more distinct, they are exhibited upon an enlarged scale. The concha, with the tube leading inwards from it marked a, terminates at the inner end, as already stated, in the tense membrane of the tympanum placed obliquely to the axis of the tube. The resemblance of this tube and the concha to the speaking or hearing trumpet is evident, and the physical purposes which it fulfils are obviously the same, being those of collecting and conducting the sonorous undulations to the membrane of the tympanum, which will vibrate sympathetically with them. 945. The Middle Ear is a cavity surrounded by walls of bone, which, however, are removed in fig. 435, to render visible its internal structure. An opening corresponding to the membrane of the tympanum is made in the external wall, and the external part of the inner ear shown in the figure is part of its inner wall. The inner and outer walls of this cavity are very close together ; but the cavity measures, vertically as well as horizontally, about half an inch, so that it may be regarded as resembling the sounding-board of a musical instrument, composed of two flat surfaces, placed close and nearly parallel to each other, the superficial extent of which is considerable compared with their distance asunder. 946. This cavity is kept constantly filled with air, which enters it through a tube, b, called the Eustachian tube, opening into the pharynx, and forming part of the respiratory passages behind the mouth. Without such a means of keeping the cavity supplied with air, having a pressure always equal to that of the atmosphere, one or other of two injuries must have ensued ; either the air in the cavity, having a temperature considerably above that of the external air, would acquire a proportionally increased pressure, which would give undue tension to the membrane of the tympanum, and perhaps rupture it, or the air confined in the cavity would be gradually absorbed - by its walls, and would consequently be rarefied, in which case the pressure of the external atmosphere, being greater than that of the air in the cavity, would force the membrane of the tympanum inward, and ultimately break it. By means of the Eustachian tube, however, a permanent equilibrium is main- tained between the air in the cavity and the external air, just as is the case in a drum, or in the sounding-board of a musical instrument, where apertures are always provided to form a free communication with the external air. 947. In the inner wall of this cavity there are two prin- Q Q 594 ANIMAL PHYSICS. cipal foramina, a greater and a lesser ; the former being called, from its oval shape, the fenestra ovalis, and the latter the fenestra rotunda ; the former is shown at /, in fig. 435, and the latter at o. Over both of these, elastic membranes are tightly stretched, as the membrane of the tympanum is over the inner end of the external meatus. Between the membrane of the tympanum and the membrane of the fenestra ovalis there is a chain, consisting of three, and in the young of four, small bones articulated together, and moved by muscles having their origin in the bones which form the walls of the cavity. Three of these hones are shown in fig. 435, at d, e, and /. The first, d, is called, from its form, the malleus, or hammer ; the end of its handle is attached to the membrane of the tympanum near its centre ; its head, which is round, is inserted in a corresponding cavity of the second bone, e, called the incus, or anvil ; and the smaller end projecting from this, articulated with the third bone, /, called the stapes, or stirrup, from the obvious analogy of its form; in the young individual a fourth bone, orbicidare, occurs between the two last fig. 437 c. The base of this stirrup corresponds in magnitude and form with the fenestra ovalis, in which it is inserted, keeping, as it would appear, the membrane which covers that aperture in a certain state of tension upon it. The handle of the malleus being firmly attached to the centre of the membrane of the tympanum, draws that membrane inwards, so as to render it more or less convex, or rather conical, towards the tympanic cavity. The muscles which act upon these small bones are supposed to have the property of giving greater or less tension to the two membranes which they connect, so as to render them more or less sensitive to the sonorous undu- lations propagated through the external ear. When the sounds are loud the muscles render the membranes less sensitive, and when they are low they render them more so. According to this supposition, when we listen attentively to low sounds, we not only concentrate the attention of the mind upon them, but we also act upon the nerves which govern the muscles inserted in the chain of auricular bones, and thereby increase the sensitiveness of the organ. It must be observed, however, that this is a mere hypothesis, no such action of these bones and muscles having been esta- blished as a matter of fact. 948. The use of the auricular bones is supposed to be the transmission of the pulsations imparted by the sonorous undu- lations from the membrane of the tympanum to the membrane of the fenestra ovalis. It has been ascertained, however, that if the membrane of the tympanum were altogether destroyed, the sense of hearing would still remain, though it would not be so perfect. It must therefore be inferred that the auricular bones aro not the only means of transmitting the sonorous INTERNAL EAR. 595 undulations to the internal ear, the air contained in the middle ear being itself sufficient for that purpose. It cannot be doubted that the membrane which covers the fenestra rotunda has some share in producing the sensation of sound ; and, if so, the chain of bones can have no effect upon it, the undulations being merely propagated to it by the air contained in the middle ear. 949. The Internal Ear. — We now come to consider the in- ternal ear, which is, in fact, the true and only organ of the sense of hearing, the external and middle ears being merely accessories by which the sonorous undulations are propagated to the fluids included in the cavities of the internal ear. The internal ear is a most curious, and, as it must be acknow- ledged, unintelligible organ, also called, from its complicated structure, the labyrinth. Its channels and cavities are curved and excavated in the hardest mass of bone found in the whole body, called the petrous or bony part of the skull. It is shown in fig. 436, as if all the surrounding mass of bone except that which forms the immediate surfaces of the cavities were cut away. 950. The Labyrinth consists of three distinct parts, called severally the vestibule, the semicircular canals, and the cochlea. The vestibule is a central chamber excavated in the petrous bone. In its external wall the fenestra ovalis, /, is formed, and the auditory nerve, n, enters through a foramen in its internal wall. At the posterior and upper part of this vestibule, are the semicircular canals, consisting of three tabular cavities bent into forms from which their name is taken, and distinguished by anatomists as the interior, posterior, and superior canals, according to their relative positions. On the interior and anterior side of the vestibule, and near the fenestra rotunda, is the cochlea, consisting of a cavity carved in the bone in the form of a spiral tube, — the name cochlea being given to it from its resemblance to the cavity within the shell of a snail, cochlea being the Latin name of that animal. Both the semicircular canals and ithe cochlea are in free communication with the vestibule. 951. The auditory nerve arrives at the bony wall of the internal ear through a passage called by anatomists the internal auditory meatus. Before entering the foramen provided for its admission into the internal ear, it separates into two principal branches, one of which is directed to the vestibule and the other to the cochlea, which are thence called respectively, the vestibular and cochlear nerves. Within the three semicircular canals are included flexiblo Q Q 2 596 ANIMAL PHYSICS. membranous tubes of tbe same form, called the membrarums canals. These include within them the branches of the auditory nerve, which pass through the semicircular canals, and they are distended by a specific licpiid called endolymph, in which the nervous fibres are bathed. The bony canals around these membranous canals are filled with another liquid, called perilymph, which also fills the cavities of the vestibule and the cochlea. It appears, therefore, that all the cavities of the internal ear are filled with liquid, and it must, accordingly, be by this liquid that the sonorous undulations are propagated to the fibres of the auditory nerves. The liquid being incompres- sible, the pulsations impax-ted either by the axiricular chain of bones, or by the air included in the cavity of the middle ear, or by both of these, to the membranes which cover the fenestra ovalis and the fenestra rotunda, are received by the liquid perilymph within these membi’anes, and propagated by it and the endolymph to the various fibres of the auditory nerve. This arrangement will be rendex'ed more clearly intelligible by reference to fig. 436, which is a perspective magnified view of the labyrinth, — the canals, vestibule, and cochlea being laid open so as to display their interior. Fig. 43G. Th e four minute bones already described as connecting the membrane of the tympanum with the fenestra ovalis, are shown separately, in their natural size, at a, 1, c, d, in fig. 439, and on an enlarged scale in fig. 438. INTERNAL EAR. 597 952. The form of the membrane of the tympanum has been generally considered by anatomists as elliptical. Professor Sappey has submitted a considerable number of cases to measure- ment, either by means of moulds, by a fresh preparation, or by those preserved in the museum of the Faculty of Medicine in Paris, and has found that the form, if elliptical at all, is so slightly oval as not to be distinguishable from a circle except by very exact measurement. He found that the proportion of two diameters was about that of 20 to 21. A d. Stapes. Fig. 438. BOSES OF A TYMPANUM ENLARGED (Arnold), surface a'leU8’ head ’ 2’ hantile • 3. loug process ; 4, short process ; 5, articular B. Incus. 1. body ; 2, long process ; 3, short process ; 4, articular surface. C. btapes. 1, head ; 2, posterior crus ; 3, autex-ior cins ; 4, base. C*. Base of stapes. ’ I). The three bones in their natural position. The same bones are shown, in connection with the muscles which move them, in fig. 439 ; where a a. is the tympanic cavity, b the circle to which that membrane is attached, c the handle of the malleus resting on the middle of the membrane, d the head of the malleus articulated with the incus, g, and e the handle of the malleus passing into a cavity called the glenoid fissure, / the internal muscle of the malleus, h the orbicular bone i the stapes, h the muscle of the stapes. The two muscles / and lc are the provisions by which the necessary tension is imparted to the two membranes. 3 The membrane of the tympanum is concave outwards in man and other mammalia, but convex in birds. Sometimes the 598 ANIMAL PHYSICS. concavity is confined to its central part, but in general extend- to its edges, its shape being d that of a cone rounded at its summit, the diameter of whose base is four-tenths, and whose height is four- fiftieths of an inch. c 953. The Auditory Nerve being confined to the laby- rinth, it is evident that the external and middle ears, with their appendages, can ' serve no other purposes than that of transmitting and a b c u augmenting the force of the Fig pulsations of the external ear, on the principle of the ear-trumpet. The terminal filaments of the auditory nerve are diffused through all the intricate cavities of the labyrinth, where they receive the action of the aerial pulsations transmitted to them. 954. Anatomists consider the labyrinth as consisting of two parts distinct in their functions, but very similar in their form, contained one within the other ; one serving as a sheath for the protection of the other, which is the sensitive part. The Osseous, or Bony Labyrinth, is composed of the hardest part of the petrous bone, and is perfectly smooth on its inner surface, every part being moulded to the form of the sensitive part which it is desired to protect. The Membranous Labyrinth is included within the osseous labyrinth, and includes within it the filaments of the auditory nerve, which enter through several foramina in the bony labyrinth from the internal auditory meatus. The external form of the bony labyrinth is shown in its natural size in fig. 440, and magnified, so as to render its parts more distinctly visible, in fig. 441. 955. Semicircular Canals Although the term semicircular has been applied to the three canals, their forms do not correspond to semicircles. They are irregularly curved, and their extremities approach each other so as to give them more the character of complete circles than that of a semicircle. At one extremity each of these tubes is considerably enlarged, forming a sort of ball, as shown at the parts marked *. In some species, however, the forms of these parts more Sappey, vol. ii. p. 555. INTERNAL EAR. 599 resemble the wide end of a trumpet. The other extremities have no such enlargement. At both extremities they open into the vestibule. I wo ot them, marked 3 and 5 in the figure, unite at their narrow extremities, so Fig, 441. ditto magnified (Sommerring). 1. Vestibule. j 2. Fenestra ovalis. 3, 4, 5. Semicircular canals. 9. Fenestra rotunda. 6, 7, 8. Cochlea. as to enter the vestibule by a common embouchure. They communicate, therefore, with the vestibule by five embouchures, three of which lead from the three ampulla? — as the enlarged ends of the semicircular tubes are called — and the two others from the small end of the tube 4, and the com- mon extremity of the tubes 3 and 5. The lengths of these canals are subject to great variation in different individuals. The longest is that marked 5, called the posterior. This generally measures in its total length from to A of an inch. That marked 3, called the superior, measures from ^ to A, and that marked 4, called the extenor, very little less than A. Individual cases, nevertheless, have been found in which they are much less developed, the length of the longest of them sometimes not exceeding a or A 0f an inch. The general calibre of these tubes is from j*m to T"r, of an inch ; the transverse diameters of their ampulla? being about twice that magnitude. 956. The Cochlea. — It is difficult to convey an exact notion of the form of the helical tube, called the cochlea. The best way to obtain a clear conception of it is to imagine, in the first instance, a flexible tube of gradually decreasing calibre, wide at one end and tapering to a comparatively small orifice at the other. Let a solid cone be then imagined, having an obtuse angle and rounded off at the apex, and let the flexible tapering tube, applied with its wide end at the base of the cone, be rolled spirally round it, so as to describe about two coils and a half in passing from the base to the apex. Now if the 600 ANIMAL PHYSICS. solid cone be imagined to consist of the more porous and spongy part of t he cranial bone, and the tube thus coiled round it of the harder and —mp brittle part, a tolerably exact notion may be obtained of the substance and form of the cochlea. A circumstance must, however, be noted in its structure, which, to simplify the explanation, we have for the moment omitted. The tapering flexible tube here ima- gined is divided throughout its entire length by a thin lamina, which runs along its diameter from end to end, so that a section of the tube made at any point would be a circle diametri- cally divided into two semi- circles ; and, in fact, the tapering tube is divided throughout its entire length into two parts, each of which, considered alone, would be a tapering semi circular tube, having the flat lamina as its base. In coiling the tube round the cone, it must also be understood that the lamina which thus divides it has everywhere its edge presented to the surface of the cone, so that in passing round the cone it would have the same form and position as the thread of a conical screw. (Fig. 442.) This spiral lamina is formed of bony matter through the extent of about § of its entire width, measured from its inner edge outwards ; the remaining 3, extending to the outer edge of the spiral tube, being membranous. 957. The Vestibule. — The cavity of the vestibule is irregularly formed, its extreme length being from -SB to its breadth from ^ to too. and its thickness from Jfj to of an inch. 958. The Membranous Vestibule consists of two vesicles, or sacs, — the superior of ovoidal form, called the utricle; and the inferior and smaller, of spheroidal form, the sacculus. These, though apparently distinct, are closely connected together. The parts of the membranous labyrinth which are deposited iu the semicircular canals are membranous tubes, which correspond with these canals in form, having a diameter about J of that of the bony tubes through which they pass. They all open into the utricle by four orifices, one of which is common to two of them. Like the bony canals, they have enlargements at one end, which nearly fill the bony cases. 959. Endolymph and Otoliths. — The utricle, sacculus, and the semicircular membranous tubes communicating with them, are filled with endolymph, and the utricle and sacculus also contain two small rounded solids, called otoliths, which consist of carbonate of lime agglutinated together by mucous and animal matter.* 960. Perilymph. — The whole extent of the cavities of the vestibule, and the canals outside the membranous sacs and tubes here described, as well as those of the cochlea, are filled with perilymph. In this the membranous sacs and tubes float, without being connected with the bony Quain, Sixth Edit. vol. iii. p. 63. COCHLEA. 601 sheaths around them. The perilymph and endolymph are secreted, the one by the membrane which lines the bony tubes, and the other by that which composes the corresponding parts of the membranous labyrinth. 961. The Acoustic Nerve. — The acoustic nerve, the filaments of which enter the vestibule through foramina placed at the side opposite to the fenestra ovalis, sends branches into the utricle, the sacculus, and round the three membranous canals, over every part of which they spread them- selves. Other ramifications passing into the cochlea spread themselves over the spiral lamina, which divides that tube diametrically. The cavities of the cochlea being filled with perilymph alone, to the exclusion of endolymph, it follows that the nervous filaments there are subject to the action of a fluid different from that to which they are exposed in the vestibule and the canals, where they are bathed, not in perilymph, but in endolymph. 962. The Distribution of the Acoustic Nerve in the Cochlea is shown in the section of the cochlea and the bony cone within it in fig. 443 ; where 1 is the auditory nerve, which diverges in the manner shown in the figure, through the in- terstices of the bony cone round which the cochlea is coiled, enter- ing each coil at the point where the spiral lamina meets it, and then are spread out, as shown at 2 upon the lamina. The central nerve, which ascends to the summit of the cochlea, is shown at 3, and the branch which proceeds to the vestibule at 4. A perspective view of the spiral lamina on a magnified scale, with the filaments of the auditory nerve spread over it, and divested of the cochlea, is shown in fig. 444. 963. Having thus fully explained and illustrated the com- plicated mechanism of the ear, it would be highly satisfactory to be enabled to show how these various forms are connected with the physical laws of acoustics. All the analogies of nature must impress us with the conviction that none of these curious forms Lave been created without a purpose, and that such purpose can only be to confer the highest attainable perfection upon the sense of hearing. Philosophers have, nevertheless. Fig. 443 (Arnold). 602 ANIMAL PHYSICS. not been so happy as to discover the physical relation between Fig. 444. PERSPECTIVE VIEW OP THE SPIRAL LAMINA, WITH THE FILAMENTS OF THE AUDITORY NERVE UPON IT, DIVESTED OF THE COCHLEA (Sappev). these forms and the laws of acoustics, as they have explained those which connect the structure and substance of the eye with the physical properties of light. 964. Acoustic Properties of the External Ear. — Sound consists in vibrations imparted by sonorous bodies to the atmos- phere. These vibrations radiate from the sonorous body as a centre, and become, accordingly, more and more diffused, and therefore more and more feeble, as the distance from the sono- rous body is augmented. Like the rays of light, however, they can be collected and condensed by artificial expedients, one of which consists in receiving a great number of them in the expanded mouth of a tube, like the open end of a trumpet, and by contracting such tube gradually to a small orifice, in which all the rays entering the wide end will be collected together. The loudness of the sound at the narrow end of such a tube will be greater than its intensity at the wide end, in the same proportion as the magnitude of the wide opening is greater than that of the narrow opening. Thus, if the diameter of the wide opening be ten times that of the narrow opening, the magnitude of the former will be a hundred times that of the latter, and the 603 THEOEY OF THE EAE. loudness of the sound at the narrow opening will consequently be a hundred times greater than that at the wide opening. 965. Artificial Aids to Hearing.— Now the auricle with the external auditory meatus forms just such a tube. The auricle is the wide opening which receives and collects the rays of sound. It is ti*ue that in man all parts of the auricle, are not equally efficient for this purpose, but the concha though not perfectly so, is sufficiently large to collect the necessary number of the sonorous rays, provided the ear has the ordinary degree of sensibility. In cases where individuals lose the necessary degree of sensibility, the concha of the auricle is often enlarged by artificial means. Metallic cavities properly formed being attached to each of the ears, give artificial conchae enlarged in any desired proportion. 966. The Ear-trumpet. — If this be not sufficient, a more powerful expedient is presented in the ear-trumpet, which not only enlarges the concha, but increases the length of the audi- tory meatus. In this case, the small aperture of the ear- trumpet is reduced to such a size as to allow it to enter the opening which leads to the auditory meatus, so that when thus inserted, the auditory meatus and the tube of the trumpet ex- tending to its wide and expanded end, form one continuous tube, and the wide end becomes a great enlarged artificial concha. The loudness of the sound will thus be augmented in a propor- tion equal to that of the magnitude of the mouth of the ear- trumpet, or to the magnitude of that part of the concha which has a corresponding efficiency in the collection of the sonorous rays. 967. The External Ears of Inferior Animals are in many species more favourable for auscultation than that of the human ear. It will be evident, for example, that the ears formed like those of the horse are better adapted for the col- lection of sound. 968. The Theory of the Tympanum is understood but imperfectly. It is evident, however, that its purpose consists in the propagation of the sonorous vibrations from the external air to the membranes of the fenestras ovalis and rotunda, and it is probable that it may also have some effects which are not yet fully understood, in modifying the vibrations. It has been demonstrated by Savart, that a membrane stretched tightly over an opening, as parchment is on a tam- bourine or drumhead, will be thrown into vibration by a 604 ANIMAL PHYSICS. sound produced near it. If fine sand be sprinkled upon the parchment of a drumhead, it will be agitated and thrown into various forms, the particles jumping upwards, as if they were repelled by the parchment when a sound is produced near it. but no such effect will be found if a piece of card or board be laid upon the same opening, unless a sound of an extreme loudness be produced. It will also be found that the susceptibility of such a mem- brane to enter into vibration will vary according as its tension is varied. It is evident, therefore, that in the same manner the membrane of the tympanum will be thrown into vibration by the sonorous pulsations of the external air. These vibrations will be imparted more or less to all objects with which the tympanum is in connection, and so much the more so as these objects are more or less vibratory, and as the tension of the tympanum is more or less intense. A certain portion, therefore, of the vibratory force of the membranes of the tympanum will be imparted to all the masses of bone surrounding the middle ear, and by these to the labyrinth and through it to the auditory nerve, just as a portion of the vibratory force received by the parchment of a drumhead from the drumsticks is transferred to the parchment on the lower drumhead by the wooden sides of the drum. In like manner a certain portion of the vibratory force of the membrane of the tympanum is transmitted to the mem- brane of the fenestra ovalis by the chain of bones which connects them, just as a portion of the vibration imparted to the parch- ment of the upper drumhead is transmitted to that of the lower by the cords and braces which connect the two parchment discs. These two portions of the vibratory force, however, are as nothing compared to that which is transmitted through the air which fids the cavity of the middle ear, and indeed it may be stated that, for all practical purposes, the air thus included is the sole agent in the transmission of the sound ; and hence it is evident how important a part of the auricular mechanism is the Eustachian tube, by which the cavity of the middle ear is supplied with air. However useful the membrane of the tympanum and its appendages may be, it is not indispensable to the sense of hearing. When it is ruptured, the air in the cavity^ of the tympanum communicating freely with the air in the external ear, the pulsations of the external ear are propagated to the THEORY OP THE EAR. 605 membranes of the vestibule without other modification than such as they may receive from the concha and meatus of the external ear ; and although the sense of hearing be not as perfect as before, it will not be destroyed. 969. Use of the Tympanic Bones. — The use of the chain of bones connecting the membranes of the tympanum and fenestrse, is not perfectly understood. That their sole purpose cannot be to aid in the transmission of sound from the inner to the outer ear is sufficiently apparent. It is certain, mean- while, that by the contractile power of the muscles connected with them, these bones have the power of varying, within certain limits, the tension of the membranes which the}7 con- nect. It has, therefore, reasonably been inferred that, by thus varying their tension, these membranes are rendered more or less sensitive ; and that a protecting influence is thus brought into operation, by which sounds of excessive loudness are prevented from injuring the organ, while, on the other hand, by augmenting its sensibility sounds are rendered perceivable which would otherwise be unheard. 970. If this hypothesis be admitted, a beautiful analogy will be established between the bones of the tympanum and the iris. 971. Theory of the Labyrinth. — The sonorous vibrations imparted by the air included in the cavity of the tympanum to the membranes which are stretched over the two fenestrse, are propagated by the liquids which fill the cavities of the vestibule to the filaments of the auditory nerve. Although these fluids, like all liquids, are inelastic and incompressible, they are, never- theless, capable of propagating the sonorous vibrations imparted to the membranes in contact with these, as is practically proved by the fact that persons descending to a considerable depth in the sea by means of diving apparatuses, can hear the sounds produced above the surface. Why, however, the cavities of the labyrinth have been filled with a liquid instead of ah- or any other gas, has not been explained. It appears from the experiments of physiologists, neverthe- less, that the presence of the liquid with which the labyrinth is filled is an essential condition to the exercise of hearing. The membrane of the tympanum may be ruptured, and the chain of bones removed without producing deafness ; but if the mem- 606 ANIMAL PHYSICS. branes of the fenestra, or either of them, be ruptured so that the. perilymph escape, deafness is found to ensue. 972. Experiments on Imperfect Ears. — It would have been very satisfactory, in the absence of all explanation of the structure of the labyrinth upon the general principles of acoustics, if the uses of its several parts could have been ascer- tained by direct experiments, as has been done with relation to different parts of the brain. Of such experiments, however, we are only aware of one, which is due to M. Flourens. That physiologist ascertained that the destruction of the semi-circular canals renders the sense of hearing confused and painful, but does not destroy it. 973. Structure of the Ears of Inferior Animals. — As I have already stated, however, no physical conditions yet dis- covered have explained the peculiar form given to the laby- rinth. We know not why two different fluids are there, nor have the acoustic properties of either the utricle or the sacculus or the three semi-circular canals, or, least of all, those of the cochlea, been explained. It is generally assumed, nevertheless, that all the parts which compose the ear are necessary to the perfection of the sense of hearing, although that sense may exist in a less perfect degree in the absence of some of them. We find accordingly that, as we descend in the scale of organisation the accessories of the organ dis- appear, one by one, in animals which are less and less elevated in the series. With birds, for example, the auricle is altogether wanting, and the external ear is reduced to the auditory meatus. With that class of animals also the cochlea loses its peculiar form, and the tapering tube, instead of being coiled round a cone, is straight, and is pro- portionally shorter than with superior ani- mals, as will appear by the outline of the bony labyrinth of the barn-owl, shown in fig. 445, where 2 is the vestibule, and 3 the cochlea divested of the spiral form. 974. Reptiles. — In reptiles generally the external auditory meatus is wanting, and the ear commences with the membrane of the tympanum which is its exterior part, the structure ot the cavity of the tympanum being also simplified. THEORY OP THE EAR. 607 975. Fishes. — In most species of fishes the tympanum and its appendages are wanting, and the ear is reduced to the laby- rinth, which consists of a membranous vestibule surmounted by three semi-circular canals, and having below it a little sack which seems to supply the place of the cochlea, in which one or more calcareous bodies, termed otoliths, are suspended. The auricular apparatus is placed in the lateral part of the great cavity of the skull. 976. Mollusca. — In descending still lower in the scale of organisation all traces of the semi-circular canals and the cochlea are effaced, and the organ is reduced to a membranous vestibule, which consists of a little sack filled with a liquid in which the last fibres of the acoustic nerve are diffused. Such a vestibule appears to be an essential element of the auditory organ, never being absent so long as that organ exists at all. With mollusca also the organ of audition is reduced to the vestibule, which consists of a little vesicle placed at each side of the brain including a liquid in which, besides the terminal fibres of the acoustic nerve, are found minute solid corpuscles, which incessantly oscillate, and which are analogous to the otoliths already described. Crustacea.— In the higher forms of Crustacea, an organ of hearing has been discovered by Dr. Arthur Farre, in the second pair of antennae. 977. Insects. — Although insects do not appear to be alto- gether insensible to sound, naturalists have not discovered in their structure any special organ of hearing. The sense of hearing appears to be altogether wanting in zoophytes and other inferior species. 608 ANIMAL PHYSICS. CHAPTER XVII. VOICE. 978. Animals resort to various expedients for the production of sound. Some sounds are incidental to their organic or loco- motive movements ; such are the sounds of the heart, those of the feet or wings in locomotion, those of some moveable parts of the body striking or rubbing against others ; those of respira- tion, including sounds produced by the occasional propulsion of air through the respiratory passages, such as those attending sighing, yawning, coughing, snoring, Ac. Respiration itself, when it proceeds with a certain intensity, becomes audible, especially during sleep. Some sounds produced by similar means are voluntary, and designed by the animal as a means of communication or expression with its fellows or those of other species with which it may be associated, and therefore serving the purpose of language, as when a dog scratches its paws at the door of its master, to ask for admission. None of these noises, however, are vocal. 979. Voice is a faculty common to nearly all the superior classes of animals, and subject to greater variation, not only comparing species with species, but comparing individual with individual, than almost any other physical character. It is a sound produced by the propulsion of air through a cartila- ginous apparatus established in that part of the throat between the hyoid bone, situate at the base of the tongue, and the uppermost ring of the trachea. The position of this apparatus in the throat is shown in fig. 366, p. 438, placed in front of the upper part of the (esophagus, and lying between the posterior cavity of the mouth, called the pharynx, and the top of the trachea. 980. This vocal apparatus is called the larynx. The hyoid bone to which it is attached, is placed at the upper part of the throat, immediately below the lower jaw, and is more or less moveable. To this bone the upper part of the larynx is attached by a membranous connection, which gives a certain freedom of motion to the parts connected with it. 981. The Glottis.— The part of the larynx which is the GLOTTIS. (509 immediate agent in the production of vocal sounds, consists of two membranous pieces stretched across the opening of the windpipe, the edges of which extend from the front backwards, being inclined slightly downwards. Each of these membranes covers something less than one-half of the entire passage of the windpipe. Between their edges a cleft or fissure is left, through which the air passes to and from the lungs. This apparatus is called the glottis. Its form and structure may be illustrated by supposing two pieces of bladder or India-rubber to be stretched over the open end of a tube, as shown in fig. 446, so placed that each of them shall cover something less than half the opening, a fissure being left between them. If air be forced upwards through such a tube by means of bellows, a sound will be produced, provided the opening between the mem- branes be not too wide, and this sound may be made to imitate the voice of an animal. If expedients be adopted, which may easily be done, to vary the tension of the mem- brane, the sound will be more or less acute, according as the tension is increased or diminished. Now the glottis, as described above, being a fissure-like opening bounded by similar membranes, and the lungs having the power of propelling air through the fissure with more or less force, it follows that vocal sounds can be produced, provided that the appendages of the larynx connected with the membranes forming the glottis, are supplied with a provision by which the tension of the membranes and the breadth of the cleft or fissure between them can be varied within the necessax-y limits. When no sounds are desired to be produced, the fissure is left so open that the air in respira- tion passing through it, fails to put the elastic membranes which bound the fissure, into vibration, and consequently the aerial undulations which are the physical causes of sound, are not produced. But if by a voluntai-y action the apparatus of the larynx connected with the membranes forming the glottis, is capable of contracting the fissure, and giving the necessary tension to the membranes, then the conditions necessary to enable the expired air to throw the membranes into vibration being fulfilled, sound will be produced, the pitch of which will depend upon the tension of the membranes and the magnitude of the fissure, while the character of the sound will be influenced by a variety of other conditions depending on the form and magnitude of the passages through which the air thus put in vibration passes before it issues into the atmosphere, — that is, upon the form and magnitude of the pharynx and the other buccal and nasal passages. 982. From this brief statement it will be apparent that the larynx, combined with the lung3 and trachea below it, and the cavities of the mouth and nose above it, forms in fact a sounding or musical apparatus, having a close analogy to certain reed instru- ments, such as the bassoon or hautboy, or certain organ-pipes. The lungs and trachea perform the part of the bellows and wind- a a Fig. 446. G10 ANIMAL PHYSICS. cliest of an organ-pipe ; the larynx corresponds to the mouth- piece, or that part of the organ-pipe which gives the peculiar character to the sound ; and the mouth and nasal passages correspond to the tube of the instrument between the mouth- piece and the place from which the column of air thrown into vibration escapes into the atmosphere. These points being explained, the structure of the larynx and the purposes which it is destined to attain will be easily comprehended. 983. The Larynx is formed of four principal cartilaginous pieces called, respectively, the thyroid, cricoid, and the two arytenoid. The thyroid, so called from its shield-like form — dupebs (thureost, a shield — is the largest of the pieces composing the larynx, and consists of two flat plates, the upper edges of which are curved into the form of the letter S. They are situated in front, at an acute angle along the median line, and form a prominent projection in the throat of a man, visible on the exterior, and vulgarly called Adam's apple. They are less perceptible in the female throat, being inclined at an obtuse or a much less acute angle. The upper edges of these ate — or wings of the thyroid cartilage, as anatomists call them — have the form of the letter m placed sideways. They are separated from the hyoid bones by a broad membrane, called from its connections the thyro-hyoid membrane. The lower part of the thyroid cartilage is connected with the cricoid cartilage by another membrane called the crico-thyroid membrane. The cricoid cartilage itself lying below the thyroid, as shown in fig. 447, derives Body of Hyoid attached to base of tougue. Hyoid — . Thyro-hyoid Adam’s apple —V • 1 Thyroid Cricoid — l Posterior part in front of ( cesophagus. Fig. 447. PROFILE OF THE LARYNX. its name from its ring shape, — Kphcos (krikos), a ring. It completely surrounds the windpipe. Its lower border, being circular and horizontal, is connected with the uppermost ring of the trachea by a membrane. The upper border is curved, being much higher behind than before, the hinder part lying between the vertical edges of the ate of the thyroid, so as to fill up the space surrounding that part of the larynx. The relative position and names of these several parts of the larynx, are shown in profile in fig. 447. LARYNX. 611 984. A vertical section of the larynx made in tlie median plane is shown in fig. 448, where the several parts are indicated. It will he seen that the Hyoid- Thyroid - Cricoid - Trachea. Fig. 44S. VERTICAL SECTION OF LARYNX BY THE MEDIAN PLANES. inner surface of the hyoid bone inclines backwards, and is met by a sort of valve moveable on a hinge, called the epiglottis. In the figure this valve is represented as inclined against the hyoid bone so as to close the upper end of the larynx ; and in this position of the parts, all respiration would be stopped. This is a position, however, which the epiglottis never assumes, except during the act of deglutition. One of the uses, therefore, of the epiglottis is to close the larynx during deglutition, so as to prevent the food or drink from descending into the lungs instead of passing in its proper direction ; nevertheless, as every one has experienced, especially in drinking, it sometimes happens that the liquid, passing into the pharynx before the epiglottis has time to close the larynx, finds its way by mistake, as it were, into the larynx ; or, as is vulgarly but quite correctly said, “goes the wrong way.” If this were continued, suffocation would ensue ; but an involuntary spasmodic action instantly takes place, by which the liquid introduced is expelled in coughing, and if any remains it is soon evaporated. 985. Besides the two membranous cords forming the glottis properly so called, described above, there are two others at a small distance above them, much less considerably developed, and which, though not without some influence upon the vocal sound, have no part in its pro- duction. As the membranes of the glottis have received the name of the inferior, or true vocal cords, the latter have been called the superior, or false vocal cords. In the vertical section of the larynx shown in fig. 448, both of these cords are represented inclined slightly from the front back- wards, and including between them a space indicated by dark shading. This space is called the ventricle of the glottis. 986. The two arytenoid cartilages are immediately connected with tho true vocal cords, and also by proper muscles, as well with the thyroid as the cricoid cartilages, and, as will presently be explained, form an important part of the mechanism by which the tension and separation of the vocal cords are governed. A front view of the larynx is shown in fig. 449, the several parts being indicated as before; and the form of its entire surface, produced by a section made by a vertical plane through the centre of the larynx at right angles to the medial plane, is indicated by the dotted lines. n R 2 612 ANIMAL PHYSICS. In fig. 450 a front view of the thyroid cartilage is shown on a larger scale ; where 1 is the summit of the vertical ridge, forming what is called Inner wall, indicated by dotted lines. Superior or false vocal cords Ventricle \ Inferior or true vocal cordt Inner wall indicated by dotted lines. — Hyoid. Thyroid. Cricoid. — Trachea. Inner wall, indicated by dotted lines. Inner wall, indicated by dotted lines. Fig. 449. LARYNX VIEWED FROM THE FRONT. Adam’s apple ; 2, the right ala, or wing ; and 3 and 4, a posterior pro- jection called the cornu, or horn. In fig. 451 is shown a front view of the cricoid cartilage ; where it will he seen that the front part, 6, is very Fig. 450. THYROID CARTILAGE. FROST VIEW. Fig. 451. CRICOID CARTILAGE. FROST VIEW. narrow, while the posterior part rises so as to fill the space between the posterior edges of the thyroid. The forms of two arytenoid cartilages are shown at 7 ; but these will presently be more fully described. In fig. 452 is shown a view of the glottis and the parts immediately con- nected with it, as it would be seen if looked at downwards, in a line directed along the axis of the windpipe. The relative position and structure of the parts will be rendered more clear by figs. 453 and 454. In fig. 453 is presented a profile view of the thyroid and cricoid car- tilages, with part of the trachea. In fig. 454 is a posterior view of the larynx and part of the trachea, dissected to display the muscles. 087. It appears from experiments made with the artificial larynx, such as that described in 081, and on the natural larynx dissected ; from the dead subject as well as observations LARYNX. 613 made on cases of throat-wounds, accidentally or self-inflicted, bird’s-eye view of the interior of the larynx and of the surrounding parts (Willis). 1. Opening of the glottis. 2. 2. Arytenoid cartilages. 3. 3. Vocal cords. 4. 4. Muscles connecting the posterior parts of the cricoid with those of the arytenoid cartilages, called the posterior crico-arytenoid muscles. 5. Muscles connecting the anterior parts of the right arytenoid with the cor- responding parts of the cricoid. Similar muscles connect the left arytenoid with the left part of the cricoid, which arc removed in the figure. These are called tho lateral crico-arytenoid muscles. 6. Muscle connecting the two posterior parts of tho arytenoids, called the arytenoid muscle. 7. Muscle connecting the left arytenoid with the thyroid. A similar muscle in the right arytenoid is omitted in the figure. These are called the thyro-arytenoid muscles. 8. Upper border of the thyroid cartilage, seen by projection, being, however, at a much higher level than the vocal cords. 9. 9. Upper border and posterior part of the cricoid cartilage. 13, 13. Ligament connecting the posterior parts of the arytenoid with those of the cricoid, called the posterior crico-arytenoid ligament. on the human subject, and inflicted for the purposes of science on inferior animals ; that the glottis is the instrument — and the sole instrument — of voice. The trachea and inferior parts of the respiratory apparatus have no other share in the production of sound than the bellows, air-chest, or air-conduits, have in the production of the notes of an organ. The parts of the larynx, the buccal and the nasal cavities above and before the glottis, have a certain influence in modifying the tone and character of the vocal sounds, but have no share whatever either in their production or in determining their pitch. Muller showed that, with an artificial larynx constructed in the manner described in 981, vocal sounds could be produced, corresponding with those of the human voice, and that Fig. 452. G14 ANIMAL PHYSICS. their character could be varied by placing above the artificial Fig. 453. SIDE VIEW OP THE LARYNX (Willis). 8. Thyroid. 9. 9. Cricoid. 10. Crico-thyroid muscle. larynx tubes or cavities variously formed. It is, however, with the natural larynx dissected from the dead subject that the most conclusive and satisfactory experiments have been made. 988. These experiments, and the manner of performing them, are illustrated in the diagrams (fig. 455 and fig. 456), the former representing the arrangement by which the sonorous effects of the larynx alone, separated by the section from the parts above and below it, and the latter that by which the vocal effects of the organ with all its accessories are illustrated.* Fig. 455 represents the larynx, separated from the cartilaginous parts above and below it, fixed by its cricoid cartilage against an upright pillar. The scale c, suspended to the projecting part (() of the thyroid cartilage, is loaded with a weight which can be varied at pleasure, and which, playing the part of the crico-thyroid muscles, causes the thyroid cartilage to turn * These two diagrams have been copied by permission from Bedard. — Traiicdc Ph2sinlogic Uumainc. 11. Crico-tliyroid membrane. 12. Upper rings of trachea. LARYNX. 615 on its junction with the cricoid as upon a hinge, and thus to vary the tension of the vocal cords according to the varying weight put into the Fig. 454. TRACHEA DISSECTED (Quain). a. Right arytenoid cartilage. t, t. Posterior margins of thyroid, c. Back of cricoid. li. Hyoid bone, e. Epiglottis. b. Left posterior crico-arytenoid muscle. s. Arytenoid muscle. 1. Fibrous membrane at back of tra- chea, with glands lying on it. n. Muscular fibres of trachea. r. Cartilaginous rings of trachea. scale c. A little apparatus, a, fixed to the pillar above the larynx, is so adapted as to be capable of varying the opening of the glottis within given limits by means of weights placed in the scales b b. Air is forced through the glottis by applying a bellows to the tube cl, which in this case repre- sents the trachea, while the bellows represents the lungs. Connected with the tube d is a siphon mercurial gauge rn, indicating the pressure of the air which acts upon the glottis. In the experiments thus made, it was found that the larynx detached from the body could pro- duce all the tones corresponding to the register of the human voice, — that is, from 2 to 2£ octaves. These tones were produced after all parts of the larynx above, and before -the inferior, or true vocal cords, were removed. With every addition of weight placed in the dish c, the pitch of the note produced was raised, and with every diminution of weight it was lowered. When the fissure of the glottis was left in the state in which it remained after death, the scales b and their weights being detached, a vocal sound 616 ANIMAL PHYSICS. was still produced ; but it was a hoarse and low one, showing that the opening of the glottis was still sufficiently contracted to produce a sound, though one very low in the scale. Fig. 455. 989. That the glottis and vocal cords are the true and only instruments of voice is proved by two classes of experiments. If an incision be made in the trachea at any point below the glottis, however near to it, so that the air expired shall escape without passing through the glottis, no sound can be produced. If, on the other hand, an incision be made above the glottis, EXPERIMENTS ON TPIE LARYNX. 617 no matter how near to it, so that the air expired shall escape through it without passing through any of the superior parts, a vocal sound will he produced, differing in its tone only from that which would ensue if the air had followed its natural course through the pharyngeal, buccal, and nasal cavities. It follows, therefore, that the sole instrument of voice is the glottis, but that the passages and cavities above it modify the character of the sound produced. 990. The peculiar tone and quality of the voice is affected by an infinite number of parts, which are thrown into vibration by the vibratory movement of the glottis. Thus, the quality of the tone is not only affected by the form and magnitude of the cavities of the pharynx of the mouth and of the nasal passages, but also by the parts, solid and fluid, about the chest and head, and even by the bodies in contact with which the speaker is placed. When it is considered, therefore, how various in magnitude, form, and material are the parts of the body thus affected by the glottis, it will cease to be wonderful that scarcely two human voices can be found that have the same quality of tone, so that not only the individual identity of an unseen speaker can be determined by his voice, but, to a cer- tain extent, the age and sex are betrayed by it. The voice of an aged person differs from that of an adult, the voice of an adult from that of a child, and the voice of a woman from that of a man. 991. To illustrate the vocal functions of the larynx with all its appendages, the head and other parts detached from the dead subject are fixed to an upright piece, as shown in fig. 456. In this case, the apparatus a, shown in fig. 455, is replaced by a sort of compressor ( a a, fig. 456), acting externally upon the larynx, so as to graduate the opening of the glottis. The weights placed in the scale C act upon the thyroid cartilage, as in the former case, so as to vary the tension of the vocal cords. The tube d, connected with the lower part of the trachea, serves for the intro- duction of air, a bellows connected with it supplying the place of the lungs. In experimenting in this way it is not possible, as in the former case, to determine with precision the varying magnitude of the opening of the glottis ; but, on the other hand, it demonstrates in a striking manner the influence exercised by all the superjacent parts in swelling and giving intensity to the voice, and imparting to it the peculiar tone, quality, or timbre, which characterises the living voice. 992. In speaking, the pitch of the voice is subject to but little variation, being generally limited to half an octave. In singing, however, the register, as it is called, is much moro 018 ANIMAL PHYSICS. extensive, its limits varying with different individual-, and -rill more with different ages and sexes. The musical characters of Fig. 45G. voices are variously denominated, according to the extreme limits of then- register, proceeding from the highest to the lowest in the scale, as follows : soprano, contralto, tenor, barytone, and basso. The soprano and contralto are voices found only in females or in children. Boys’ voices — before the age of puberty — are generally contraltos. The tenor, barytone, and basso, are men’s voices. Since every individual organ differs from another, no exact limits can be assigned to these several classes of organs. One soprano will have a higher or lower register than another, and the like may be said of the other classes of REGISTER OF THE VOICE. 619 voices. The mean limits, however, for each of these classes, is indicated, as follows : the numbers annexed to Limits of register. them being the number of double vibrations of the glottis which are produced in a second of time. The production of each note is ascer- tained by the experi- mental researches of Cagnard de la Tour, Savart, and others. It appears, therefore, that the extreme limits of the vibratory power of the female glottis of a soprano singer, is 1056 and 264 double vibrations per second.* 993. Speech consists of articulated vocal sounds. Voice is com- mon to all mammifers, but speech is the pecu- liar privilege of man. Although mammifers in general possess the organs of articulation, cavities, the veil of the Double vibrations per second. SOPRANO CONTRALTO TENOR BARYTONE BASSO i . 1056 264 704 176 528 132 110 220 S2'5 consisting of the pharynx, the nasal palate, the tongue, cheeks, teeth, and * A single vibration of a pendulum consists of one movement from left to right, and a double vibration of two movements ; one from right to left, and the other from left to right. Since the lips of the glottis in vibration move alternately up and down, a double vibration consists of two such motions, one from the extreme limit upwards to the extreme limit downwards, and the other from the oxtreme limit downwards to the extremo limit upwards. Tho pitch of the note depends only on the number of vibrations per second. Its intensity or loudness, other tilings being the same, depends upon tho space through which tho vibration takes place. In calculating the numbers of vibrations in tho above table tho pitch of A in the treble is assumed to correspond to 440 double vibrations. Tho pitch of this note varies within narrow limits in different schools and orchestras in Paris and Berlin, between 4 24' 14 and 437‘32. It appears from some experiments recently mado by Mr. Donovan of Dublin, that the present coueort pitch of tho principal London orchestras is nearly 450. The superior limits of tho mean register of the soprano and tenor given above are perhaps a little too high, though many voices of this class have arisen above them. The classification of voices however, is somewhat arbitrary, and is determined by the best und most effective parts of the register as often as by its extremo limits (soo Handbook Nat. Phil Acoustics). ’ '' 620 ANIMAL PHYSICS. lips, they are nevertheless incapable, in general, of producing articulate sounds, and still more of using these for the expres- sion of thoughts or feelings. Individuals affected with idiotcy from their birth are incapable of producing any other vocal sounds than inarticulate cries, although supplied with all the internal instruments of articulation. Persons deaf and dumb are in the same situation, though from a different cause ; the one being incapable of imitation, and the other being deprived of the sense of hearing the sounds to be imitated. 994. Mammifers are capable of producing vocal sounds, which vary with different classes. Thus, the horse neigh=, the dog barks, the cat mews, the ass brays, the cow lows, the pig grunts, the lion roars, and so on. These various modifications of voice depend on the peculiar structure of the larynx, but much more upon the buccal and nasal cavities. 995. The Horse has a glottis fonned of simple vocal cords, considerably developed and surmounted on each side by ven- tricles having a wide entrance. The vocal glottis scarcely measures half the glottidal fissure, and the interarytenoid glottis is more developed than in man. The neighing is pro- duced by a succession of interrupted expiratory movements. The tension of the vocal cords gradually decreases during the continuance of the expiration ; and, consequently, the first sounds of the neighing are more acute than the last. 996. The Ass has a larynx which, like that of the horse, is supplied with only two vocal cords. The ventricles are large, but the entrance to them much smaller. The voice of the ass has a peculiar character, commencing by an inspiration, which produces an acute sound, and terminating by an expiration, producing one more grave. 997. The Ox has a larynx differing considerably from those of the solidungular classes. The glottis is short, and the vocal cords scarcely detached upon the surface of the larynx. There are no ventricles. The voice of this animal is much more imperfect than that of the horse, consisting of a moaning sound, or lowing, having a very low pitch and very little varied. 998. The Dog has the inferior vocal cords well detached VOICES OF ANIMALS. 621 and thin upon their edges. The superior cords are scarcely perceptible. The ventricles are large, but the entrance to them narrow. The voice of the animal is subject to much variation and capable of different modes of expression. Some- times it barks, sometimes growls, snarls, and often utters a sort of neighing expressive of pleasure. 999. The Cat is distinguished from other mammifers, as well as from man, by the almost equal development of the inferior and superior vocal cords. The mewing begins by a very acute sound, which becomes more and more grave as the mouth, first open, is gradually closed. The voice of the cat, like that of the dog, is endowed with a certain extent of register. Its power of producing sounds in the higher parts of the scale is especially remarkable at certain sexual periods, when its voice bears a close resemblance to that of an infant. It has not been certainly ascertained what effect the superior vocal cords of the cat produce. It is probable, however, that in this, as in other mammifers, the principal vocal organ is the inferior vocal cords. 1000. The Pig has a larynx characterised by the anterior insertion of the inferior vocal cords upon the tracheal edge of the thyroid cartilage. The arytenoid cartilages united above the superior vocal cords are merely rudimentary ; the ven- tricles, which are deep, communicate with the interior of the larynx only by a narrow fissure. The animal has two sorts of cries ; the one low, called grunting, and the other more acute, when it is goaded or provoked. Other Animals are endowed with voice, but rarely use it. Such are the stag, the rabbit, the hare, and so on. Animals which howl and are heard at night at great distances, have generally large laryngeal ventricles. Certain apes of the South American continent are especially remarkable in this respect. The howling apes which prevail in large troops in Guiana, have hyoid bones, terminated on each side by a bony enlarge- ment lodged in the process of the lower jaw. This enlargement, which is hollow, communicates with the laryngeal ventricles prolonged below the epiglottis and thyro-hyoid membrane, and gives to their voice a peculiar character. 1001. Birds have two larynxes, a superior and inferior. Tbc 622 ANIMAL PHYSICS. superior, which corresponds to the larynx of mammifers, and which is placed at the superior opening of the respiratory pas- sages in the pharynx, can only be regarded as an accessary of the voice. The thyroid, cricoid, and arytenoid cartilages are severally rudimentary. The opening by which the thyroid cartilage leads into the pharynx can be increased or diminished by muscles which are grouped round it ; but it cannot properly be called the glottis. The true larynx of birds is the inferior larynx, placed at the inferior part of the trachea, where it bifurcates. This larynx consists of several parts : an enlarge- ment, the sides of which are partly bony and partly membranous, corresponds to the inferior part of the trachea. This enlarge- ment is called the drum , and is divided at the point of junction of the branchiae by a bony transverse piece surmounted by a thin membrane of semilunar form. At the point where the two superior orifices of the bronchial tubes communicate with the drum, they are bordered each by two lips, or vocal cords, one of which is generally more developed than the other. There are besides, between the different rings of the inferior larynx, muscles more or less numerous, the function of which is to stretch the different membranous folds which they sustain. These muscles are merely rudimentary in gallinaceous birds. There is one pair of them in the eagle, vulture, buzzard, and the cuckoo, three in the parrot, and five in singing birds, such as the nightingale, the lark, the canary, Arc. These muscles have all a common insertion in the trachea, being fixed at the T G P M H A TONGUE AND GLOTTIS OF A GRANIVOROtTS BIRD. A. The Tongue. H. Hyoid Bones. M. Muscles of tho Hyoid. P. Pharynx. G. Glottis. T. Trachea. O. Gullet. other end to the first rings of the bronchial tube. Indepen- dently of these, there are other muscles charged with the function of lowering the trachea, and thus diminishing the SINGING BIRDS. 623 length of the vocal tube, the length of which can also he modified by the action of the levator muscles of the hyoid, which, as in mammifers, are connected with the thyroid car- tilage. The levators and depressors of the trachea also have some influence upon the tension of the lips of the glottis of the inferior larynx, the elevation and depression increasing and relaxing such tension. The details of these organs will be more clearly understood by reference to figs. 457, 458, and 459. That the inferior larynx is the true vocal organ of birds, is proved by the fact that the voice is not sensibly modified when an incision is made in the trachea below the superior larynx and above the inferior, while, on the other hand, very varied sounds can be produced with the inferior larynx after the superior has been entirely removed. Fig. 4oS. Fig. 459. INFERIOR I.AP.YN'X OF A CROW. VERTICAL SECTION OF THE LARYNX. In fig. 45S is represented the larynx of a crow. A B. The trachea. C. The drum formed by the interior extremity of the trachea. B. The middle bone of the trachea. F. The first ring of the bronchia separated from the third bone of tho larynx by n membranous space. G. the bronchia. D. tho proper muscles of the larynx, which have been removed on the opposite side. I. The depressor muscle of the trachea. < A vertical section of the larynx is shown in fig. 459. B. The inferior portion of the trachea cut through the middle. C. The semilunar membrane situated above the point of union of the two glottises, and fixed to the bony transverse piece, 0. A. The rim or flange formed by tho internal lip of the glottis of the right side. D. The internal face of the right bronchia formed by a tympanic membrane. E. A part of the cavity of the right bronchia laid open by tho soctiou of a part of that mcmbrano. The voice of birds is produced like that of mammifers, by the vibration of the lips of the glottis. The function of the semilunar membrane, which surmounts the bony piece of the drum, has not been satisfactorily deter- mined. It is probable, however, that it enters into vibration when vocal sounds are produced. The drum itself is the organ by which the intensity of the sound is increased, and is analogous to the laryngeal ventricles of mammifers. The varying length of the vocal tube determined by the play 624 ANIMAL PHYSICS. of the depressor and levator muscles of the trachea, have much more extent in birds than in mammifers, and no doubt produce important modifications in the modulation of the voice. 1002. Reptiles have sometimes a true voice, as is evident in the case of frogs, toads, and other amphibia. The cavity of the larynx presents upon the sides membranous folds, which, parting from the base of the arytenoid cartilages, may properly be called vocal cords. It is by these that the noise called croaking is produced. Bull frogs also exhibit on each side of the neck, under the ear, an apparatus for increasing the inten- sity of sound, consisting of an elastic membranous pocket, which opens into the mouth upon the side of the tongue, and dilates when the animal croaks. 1003. Insects in general produce sounds remarkable by then- acuteness. Since they breathe by trachea, as shown in fig. 363, they have no organ analogous to the larynx, and the noises they produce result either from the friction of some parts of the body against others, or from movements determined by the play of the muscle on certain organs, or by the larger spiracles, as in the common house-fly, humble-bee, Ac. Some insects produce noise by rubbing the dentated parts of theh- tliighs against the external border of their elytra, others by rubbing their elytra against the rings of the abdomen, or by rubbing the r-ings of the thorax one against the other. Other insects, such as grasshoppers and cicadfe, are supplied on the sides of their bodies with a little dry membrane extended upon a horny frame, to which they impart oscillation by the aid of a muscle which acts upon it in the same manner as the muscles of the chain of tympanic bones act on the ear — that is to say, by a rapid alternation of tension and relaxation. Other insects produce noises which do not depend at all upon the play of their organs, but upon the action of some of these organs on the bodies around them ; such, for example, are various insects which gnaw wood, and which strike either with their mandibles or with the hard extremity of their abdomen.* Bedard, Physiologie Humaine, p. 600. THE EGG. 625 CHAPTER XVIII. DEVELOPMENT. — MATURITY. — DECLINE. — DEATH. 1004. The Egg. — The origin of life is wrapped in mystery. Nevertheless, the phenomenon of vitality has been traced back to points that cannot be far removed from the main-spring which owes its tension to the immediate will of the Creator. Animals holding the lowest place in the scale of organisation commence their visible vitality in phenomena having a close i-elation to those of vegetation. Such modes of reproduction, however, are strictly limited to the lowest forms of organic life, all animals of superior organism originating in an egg or ovum. 1005. Oviparous and Viviparous. — In some the egg is hatched before, in others after it is laid. Although, strictly speaking, all such are therefore oviparous, that term has been limited to those classes in which incubation succeeds ovi- position. Those classes whose eggs are hatched within the maternal body are called viviparous. ■. With some oviparous animals the eggs after being laid become objects of the most tender solicitude on the part of the mother, who not only hatches them by the warmth of her own body, but tends the young after breaking the shell till they become capable of tending themselves. With others, certain insects for example, the eggs are deposited in places where they can be protected from the vicissitudes of weather and the attacks of natural enemies, and where the young on issuing from them may find a provision of suitable nourishment, and a medium corresponding to the character and functions which mark the first period of their existence. These precautions being taken, the mother abandons the egg, and often dies before the young come to life. 1000. Structure of the Egg.— Nothing can be s s more G26 ANIMAL PHYSICS. interesting and instructive than to trace the organic develop- ment from the egg to maturity and final dissolution. To follow- such an exposition, however much it may be simplified and divested of technicalities, it will, nevertheless, be necessary to be familiar with certain terms which express the forms of the parts of the egg and the succession of changes incidental to them. 1007. The Ovary is the part of the maternal body in which the egg is evolved. That of a mammifer is represented in fig. 460. 1008. Graafian Fol- licles, or vesicles, is the name given to the envelopes in which the eggs are enclosed in the ovary of mammifers. The corresponding envelopes in the ovary of birds are called ovicapsules, or orisact. When the development of the egg has attained a certain limit it bursts the envelope, and being expelled, is re- ceived into a tube called the Fallopian tube, from which it passes to another part of the organism, where it remains until the moment of transition Pie 4eo from embryonic to natural life. OVARY OF A MAMIIIFER— THE PIG (PoUCtlCt). T v ' Like most other organs, the ovaries, and their appen- dages, the Fallopian tubes, exist in pairs, symmetrically placed at either side of the median plane. In fig. 460 the Graafian follicles are shown in different stages of develop- ment. A follicle from which the egg has just escaped is shown at b, the opening having the form of a cleft, and another at a, in which it is round. The follicle is sometimes rent at the edges. 1009. The follicle is filled with a transparent yellowish liquid, analo- gous to the serum of the blood, holding in suspension a multitude of elementary granulations. Against the inner surface of the surrounding membrane, a stratum of matter of cellular structure is applied, which forms a sort of epithelial coating upon it. This is called the granular membrane. The ovum is much smaller than the follicle, and lies against its inner wall surrounded by a mass of cellular matter, called the. discus proligcrus. A theoretical section of one of the Graafian follicles in an advanced state of development is shown in figs. 461, 462. THE GRAAFIAN FOLLICLES. 627 d, Fig. 461. SECTION OF GRAAFIAN FOLLICLE AND OVUM (Bedard). a, is the external coating of the follicle ; b, the granular membrane within it ; e, the ovum ; d, the vitelline membrane, called also the zona pellucida ; e, e, the discus proligerus. Fig. 462. SECTION OF THE GRAAFIAN FOLLICLE AND OVUM OF MAMMIFER (Mllller). 1, is the stroma or tissue of the ovary ; 2 and 3, the external and internal tunics of the follicle ; 4, the cavity of the follicle ; 6, the vitelline membrane, being the tunic of the ovum; 6, the yolk; 7, the germinal vesicle; 8, the germinal spot. S S 2 G28 ANIMAL PHYSICS. In fig. 463, the ovum of the sow is shown, in which 1, is the germinal spot ; 2, the germinal vesicle ; 3, the yolk ; 4, the pellucid zone, surrounding the yolk, 5, the granular coat ; and, 6, a mass of adherent granules or cells. Fig. 463. 1010. Magnitude of Egg. — The general form and arrangement of the parts of the egg being thus explained by figures magnified on a great scale, it will be useful to indicate the real magnitudes of the eggs and their com- ponent parts. These vary more or less in different families of mammi- fers, but, as will be seen by the fol- lowing table,* they are always micro- scopic. Measurement in Animal. parts of a Paris inch. Observer. r Man 240 ^0 120 Bischofif. Diameter of mature ovum . J Rabbit l Ditto. Bitch 147 142 Ditto. ■ Man 1 2500 Ditto. Thickness of zona pellucitla . - Rabbit 1 1- 2500 1250 Ditto. . Bitch Togo to I050 Ditto. Germinal vesicle Rabbit Bitch . . . . . l 100 Ditto. Germinal spot r Bitch 1 2500 Ditto. Mammaliagenerally 3000 t° 2400 Wagner. Large yolk globules .... Ditto 1 1. 1 OOOO 4000 Henle. The first change which the ovum sustains after passing from the ovary into the oviduct is the disappearance of the germinal vesicle which surrounds the germinal spot. This change may be produced even before the ovum escapes from the Graafian vesicle and enters the Fallopian tube. 1011. Issuing from the Graafian vesicle the ovum is sur- rounded by the cellular mass called the discus proligerus (c, fig. 4G1). This, however, is dissolved by degrees, aud soon disappears. In passing through the Fallopian tube, it is sur- rounded by an albuminous coating, which has but little thickness. 1012. Segmentation of the Yolk.— Embryonic development * Supplement to Muller’s Physiology by Drs. Baly aud Kirkes, p. 35. SEGMENTATION OF THE YOLK. 629 is preceded by tbe remarkable phenomenon known as the segmentation of the yolk, which takes place in the following manner. Fir, ST STAGE OF SEGMENTATION OF EGG OF MAMM1FER. SECOND STAGE OF SEGMENTATION OF EGG OP MAMMIFER. In the midst of the yolk appears a nucleus having a nucleolus. This seems to exert a central attraction upon the surrounding mass, and causes clear space between it and the the yolk to contract, so as to leave a vitelline membrane. The sphere thus formed is the first sphere of segmenta- tion. The central nucleus now divides into two, which take separate positions in the centre of each hemisphere of the yolk, and which, exercising a central attrac- tion on the surrounding matter, cause the first sphere of segmentation to resolve itself into two, as shown in fig. 464. The two nuclei thus produced are then again subdivided each into two, which, operating by like central attraction, resolve the yolk into four, as shown in fig. 465. In the same manner, the resolution of each nucleus thus successively pro- duced into two, causes the yolk to be successively divided into 8, 16, 32, kc., parts. As the division is thus continued, the spheres of segmen- tation are multiplied in number, and proportionally diminished in magnitude. Fig. 466. COMPLETE SEGMENTATION OF EGO OF MAMMIFER. 1013. The phenomenon here described is called complete segmentation, inasmuch as it affects the entire mass of the yolk. In some animals, however, among which birds are included, a part of the yolk only, called the cicatricula, is subject to this segmentation, but in other respects the phenomena are the same. In the comparison of the embryonic phenomena of birds and mammifers, it must therefore be remembered that the cica- C30 ANIMAL PHYSICS. tricula of the bird’s egg is the analogue of the entire yolk of the mammifers. 1014. By the process of successive segmentation the surface of the egg assumes the external appearance of a blackberry or mul- berry, but when that process is carried to its extreme limit the smoothness of the surface is restored by the indefinite minute- ness of the spheres of segmentation. 1015. When the segmentation has arrived at its limit, the spheres assume the form and character of true cells, being composed of a membranous envelope with liquid and granular contents, and an internal nucleus. According as they are formed, they are pushed towards the external mem- brane of the egg by the pressure of the albuminous liquid contained within it. Their forms are changed by the pressure to which they are subject, and finally this pressure reduces the masses of cellidar matter to a membrane surrounding the yolk lying against the vitelline membrane, so that the egg is enclosed in two membranes, one within the other, the vitelline membrane being the external, and the new membrane produced as here described the internal. 1016. Blastoderm or Germinal Membrane. — This new mem- brane is called the blastoderm or germinal membrane. The liquid it includes is albuminous, with granular particles floating in it. 1017. Germ. — A dark and opaque spot now appears at a certain part upon this germinal membrane, which proves to be the first vestige of the future embryo. This spot is called accordingly the embryonic or germinal spot. While these phenomena are being developed, the egg passes through the Fallopian tube, and its dimensions are gradually increased. Its diameter, which, on leaving the ovary, is scarcely more than the 200th of an inch, is increased on leaving the Fallopian tube to the 50th of an inch. A rapid series of changes takes place in the egg after it leaves the Fallopian tube. The embryonic spot, at first circular, is elongated, and becomes elliptical. A bright line appears along its centre, in the midst of which is traced indications of the spinal marrow. Meanwhile, the germinal membrane, which was at first simple, is resolved into two lamina?, one within the other, the external being called the serous or cutaneous, and GERMINAL MEMBRANE — BLASTEMA. G31 the internal the mucous lamina. The former supplies the materials, out of which the tegumentary envelope, or external skin of the future animal will be formed ; and the latter the mucous membrane, or skin which lines its internal cavities. Some physiologists have maintained that the external lamina supplies the matter out of which the bones, muscles, and organs of sense are formed. According to Reichert, however, whose account we follow, the superficial lamina is limited to the formation of the skin. 1018. Blastema. — Between the two laminae of the germinal membrane appears the primitive blastema, from which all the organs of the embryo will be developed. From this blastema the vessels which will soon overspread the mucous lamina are produced. It is to the assemblage of these vessels that the name of the vascular membrane has been applied, although it has none of the characteristics of a true membrane. i 1019. First Appearance of the Embryo. — While the ger- minal membrane is thus resolved into two laminm, the embry- onic spot, elongated as already described into the form of an oblong ellipse, increases in thickness, and forms a ridge upon the external surface of the germinal membrane. Its extre- mities as well as its sides are bent so as to present a concave surface towards the centre of the egg, and to give the whole the form of a boat, the keel of which is presented outwards. One end of this boat, representing the stern, is greater than the other, which represents the bow. The former called the cephalic extremity, corresponds to the head of the future embryo, and the latter, the caudal extremity, to the lower part of the body. The sides of the boat-shaped mass gradually approach each other so as to convert the whole into a sort of tube with a longitudinal opening in it from end to end. This corresponds to the thoracic and abdominal cavity of the future animal. The space between the edges which thus approach each other corresponds to the umbilical opening. In the mass thus formed, the traces of the spinal marrow, the brain, and the vertebne are progressively developed. According as the embryo is gradually bent at its ends and sides into the boat-form here desciibed, the parts of the cutaneous lamina placed upon its limits are raised all around 632 ANIMAL PHYSICS. it. This elevation is most apparent at first towards the cephalic and caudal extremities. Fig. 467. mammifer’s egg about the tenth day of EMBRYONIC LIFE (Bedard). а. Vitelline membrane covered externally with viilosities. б. Cutaneous lamina of germinal membrane. c. Mucous lamina of ger- minal membrane. d. Embryonic mass. 6' The first elevation of the cutaneous membrane by the cephalic and caudal ex- tremities of the embryo. The result of the succession of changes here described is illustrated in fig. 467. As the progress of development advances, the cutaneous membrane bends itself over the cephalic and caudal extremities of the embryo, and passes under its dorsal part, forming two folds, the edges of which graduallv approach each other. This change is illustrated in fig. 46S. а. Vitelline membrane. б. Cutaneous lamina of germinal membrane. b' b'. Edges of folds of the cutaneous lamina ap- proaching each other. b'' b". Cephalic and caudal hoods formed by the cutaneous membrane bout over the extremities of the embryo. c. Mucous lamina of the germinal membrane retiring from tho cuta- neous lamina, and form- ing the umbilical vesicle. Fig. 468. OVUM AT A LATER PERIOD OF DEVELOPMENT (Bedard) 1020. Umbilical Vesicle. — According as tlie embryo curves FIRST FORMATION OF THE EMBRYO. 033 itself so as to increase the concavity directed towards the centre of the egg, the mucous lamina enters the cavity of the embryo, adapting itself to its form as shown in fig. 468. The part of this lamina which enters the cavity, is destined eventually to form the intestinal canal, while the larger portion, which is outside ( b " b"), forms a part called the umbilical vesicle, which disappears before the expiration of the embryonic period, as will presently be shown. 1021. Amnion. — The folds of the cutaneous lamina which are bent under the dorsal part of the embryo, ultimately unite and form a membrane enclosing it, called the amnion. The junction of the edges ( b ' b') takes place from the twentieth to the twenty-fifth day of embryonic life in animals whose mean embryonic period is forty weeks,* and the partition which at first exists at the point of junction soon disappears. The remainder of the lamina thus separated from that which forms the amnion attaches itself to the vitelline membrane, and is incorporated with it, forming the general coating of the egg, now called the chorion, the surface of which is covered over with villosities as indicated in fig. 468. At first the amnion surrounds only the dorsal and terminal parts of the embryo, as shown in fig. 468, but as the umbilical orifice is more and more contracted by the mutual approach of the ventral plates, the amnion, drawn together by the edges, encloses more and more of the embryo. 1022. Allantois. — The mucous lamina, as already stated, adapting itself to the cavity of the embryo, contracts its width at the lower part, as shown in fig. 468 ; but as the develop- ment proceeds, this contraction is rapidly increased, and about the twenty-fifth day of embryonic life it is reduced to a com- paratively narrow duct, forming a communication between the concavity of the embryo and the external part of the umbilical vesicle. Meanwhile another development of the mucous lamina takes place near the caudal extremity, which is called the allantois, and which corresponds with the future urinary vesicle. These changes are illustrated in fig. 469. 1023. Water of the Amnion — The amnion soon after its formation is filled with liquid, probably secreted like that of * Unless otherwise expressed, wo shall take the whole embryonic period as forty weeks. * 634 ANIMAL PHYSICS. serous membranes, in which the embryo floats until the trarn-i- tion from embryonic to natural life. This liquid, which is called the water of the amnion, consists of ninety-nine per cent, of water, combined with a small proportion of albumen, chloride of sodium, phosphate and sulphate of lime, and other salt-. It continues to increase in quantity till the fifth month, at which epoch it is about equal to the weight of the embryo. After this, the quantity remaining the same, the embryo increases in volume. At the moment of transition from embry- onic to natural life, the liquid is discharged by the rupture of the amnion, and is then from one to two pints in quantity. a, b. Chorion, com- posed of the union of the vitelline membrane and cutaneous lamina. b‘, b'. Folds of the cu- taneous lamina closing to form the amnion. c. c. The umbilical vesicle with its blood- 1 vessels. d. . Cephalic part of embryo. d'. Caudal part of em- bryo. c'. Allantois, with its blood-vessels. c". First vestiges of the intestinal cavity of the embryo, formed by part of the mucous lamina communicating with the extra -cmbryo- Fig. 469. nic part of the umbilical mammifer’s ego about the twenty-fifth day of vesicle, c, c, by a narrow embryonic life (Beclard). neck. The narrow neck connecting the umbilical cavity with the umbilical vesicle is called the omphalo-mesenteric duet, and the blood-vessels distributed over the vesicle are called the omphalo-mesenteric vessels. In like manner the blood-vessels distributed over the allantois (c' fig. 469), are called the allantoid vessels. These, at a later period, become the umbilical arteries and vein. The umbilical vesicle is only a temporary organ, which dis- appears at a subsequent epoch of cmbiyonic life, the omphalo- mesenteric duct closing, and the vesicle wasting away. Before this, it contributes to the circulation of the embryo. At the close of the first month of embryonic life it fills the chief part of the interior of the egg, and has then a magnitude equal to that of a pea. About this period, the omphalo-mesenteric AMNION — ALLANTOIS. 635 duct closes, and the vesicle disappears by re-sorption in pro- portion as the egg increases. While the umbilical vesicle decreases and wastes away, the allantois increases rapidly, until it completely fills the egg, coming into contact with every part of the chorion. This state of the egg is illustrated in fig. 47 0. The development of the allantois is very rapid. At the moment when the umbilical contraction of the embryo reduces the communication between the intestine and the umbilical vesicle to a narrow duct, the allantois is contracted immediately c ' / Fig. 470. MAMMIFER’s EGG AT THE CL03E of the first month of embryonic life (Ueelurd). a, b. Chorion. 6', b\ Union of the folds of the cutaneous lamina. 6", b". Cephalic and caudal hoods. c. Umbilical vesicle decreasing in magnitude. c% c\ e, c. Allantois increased so as to fill tho egg in contact with the chorion. c", c". Incipient intestine of tho embryo. <1. Cephalic extremity. d'. Caudal extremity. above the caudal part of the embryo, and is thus divided into two unequal parts, separated by a narrow intermediate duct. The part of the vesicle included within tho duct, and situated G36 ANIMAL PHYSICS. consequently in the abdomen of the embryo, is intended at a later period to form the urinary bladder, and the part exterior to the same narrow duct, and above it in the figure, constitutes the allantois properly so called. When the allantois spreads over the internal surface of the chorion, it is gradually incorporated with it. Some physio- logists maintain that the chorion is not, properly speaking, formed by the combination of the successive membranes here mentioned, but that each atrophises and wastes away, when that which is within it is formed. According to Coste, the first chorion consists of the vitelline membrane ; the second of the cutaneous lamina of the germinal membrane, which, at first incorporated with the vitelline, ends by replacing it ; the third chorion, that which is definitive and permanent, being con- stituted of the allantoid vesicle, which remains after the second chorion disappears by atrophy. 1024. Placenta. — Soon after the egg issues from theFallopian tube, the external surface of the chorion is covered with villo- sities, which begin to be crowded with blood-vessels about the moment at which the membrane of the allantois is applied to its internal surface. These villosities at a later period disappear by atrophy at all parts, except a certain space called the placenta, indicated in fig. 469. At this part, the villosity, instead of disappearing, increases, and is considerably developed. These changes are completed about the end of the third em- bryonic month, when the part of the chorion called the placenta alone remains vascular. 1025. Maternal and Embryonic Placentas. — In immediate contact with the villosity of the placenta here described is a corresponding vascular arrangement in the maternal organism, the latter being distinguished as the maternal and the former as the embryonic placenta. The villosities of the foetal placenta (in the human species) are received in tufts into depressions in the walls of the vessels of the maternal placenta, which are enlarged into cavities or reservoirs of blood. By this admirable provision, the functions of the maternal circulation are shared by those of the embryonic, an interchange taking place between the maternal and embryonic blood through the coats of the vas- cular structure of the two placentas by endosmose, so that the embryonic blood is arterialised and nourished by the mediate action of the maternal pulmonary and digestive organs. UMBILICAL VESICLE — PLACENTA. 037 Since the placenta, therefore, discharges the functions of nutrition and respiration in the embryonic economy, it may he expected that its magnitude shall bear some relation to that of the embryo. It is accordingly found that it increases with the increase of the embryo, and maintains with the maternal organism connections more and more numerous according as the growth of the embryo proceeds. As embryonic life advances, the allantois, being converted into a twisted cord called the umbilical cord , forms the vascular connection between the embryo and the placenta, consisting, as will presently appear, of two arteries and a single vein, called the umbilical arteries and the umbilical vein , having their terminations in the villosi- ties of the embryonic placenta. The membrane of the amnion, also, after it has advanced from the cephalic and caudal extre- mities of the embryo towards the umbilical region, attaches itself upon the umbilical cord, of which it forms a sort of sheath which continues to the termination of the embryonic period. The manner in which the umbilical arteries and vein contribute to the circulation of the embryo will be presently explained. 1026. Development of the Embryo. — While the succession of changes here described are taking place in the umbilical vesicle, the allantois, the amnion, and other appendages of the embryo, the body of the embryo itself is gradually assuming the organised form proper to the future animal. The embryonic spot rapidly increasing, first assumes the form of the embryo ; the several tissues and organs then begin to be formed and developed, and in due time the embryo, attaining a certain degree of organic perfection, emerges from its embryonic state into that of natural life. The duration of the embryonic period varies in different species, as will be presently explained. The rudiments of the cerebro-spinal nervous system of the future animal are the first indications of organic form, traced upon the germinal spot at a moment when this spot consists of nothing more than a stratum of cellular structure interposed between the cutaneous and mucous laminar of the germinal membrane. This cellular mass presents along its centre a clear line, surrounded at either side and at its ends by two salient ridges, which arise from the accumulation of blastema on the borders of the primitive line (fig. 471). 1027. Formation of Spinal Cord. — The primitive line indi- cated in the figure, and the dark margin which surrounds it, fonn G38 ANIMAL PHYSICS. a sort of hollow groove, the bottom of which is represented by the bright part. The obscure borders, called the dorsal lamina z, become more and more curved, so that their edges approaching each other, are at length united along the median line, convert- ing what was first a hollow groove into a hollow tube. This tube is the vertebral canal of the embryo. A nervous laye. , which forms the inner part of the dorsal laminae, is fashioned into an inner tube which is the rudiment of the brain and spinal cord. Fig. 471. PART OF THE GERMINAL MEMBRANE OF THE OVUM OF THE DOG, SHOWING THE AREA PELLCCIDA AND RUDIMENTS OF THE EMBRYO, MAGNIFIED TEN DIAMETERS (Bischoff). A. Germinal membrane. B. Area vasculusa. C. Area pellucida. D. Dorsal laminae. E. The primitive groove boundod laterally by the pale pellucid substance cf which the central nervous system is composed. 1028. The Brain. — At its cephalic or anterior extremity the primitive nervous system has a slight enlargement, which is the first vestige of the future brain. Upon this, according as it is developed, are traced three distinct enlargements, called the cerebral cells, separated one from another by contractions. These, increasing unequally in the progress of development, sub- sequently produce different parts of the encephalon (fig. 472). FORMATION OF CERE BRO -SPIN AL SYSTEM. 639 The primitive groove (a), is not yet closed, and at its cephalic extremity the three dilatations (b) here mentioned are indicated. These correspond to three divisions or vesicles of the brain. At its caudal extremity the groove presents a lancet-shaped dilatation (c), called the sinus rhomboidalis. The margins of the groove consist of a clear, pellucid, nervous substance, Fig. 472. PART OF THE GERMINAL MEMBRANE OF THE DOG IN A MORE ADVANCED STAGE OF DEVELOPMENT (Bischoff). and along its bottom is traced a faint streak, which is probably the chorda dorsalis, or incipient centre of the vertebral column. The vertebral plates (d), are arranged on either side. The anterior of these three cells, increas- ing rapidly in volume, produces severally and successively the cerebral hemispheres, the lateral ventricles, (320), the optic thalami (fig. 241a), corpus callosum l fig. 241), and the fornix (fig. 2416). The development of these important parts of the nervous system commences about the end of the first month. Towards the fourth month, all the parts are clearly traced, and the cerebral lobes, which continue to increase, soon cover the pos- terior parts of the encephalon which are evolved from the middle and posterior cells (b). Thus, about the fifth month the quadri- geminal bodies (333), and about the seventh the cerebellum (331), are covered by the cerebral hemispheres. It is about the G40 ANIMAL PHYSICS. fourth month that the convolutions and anfractuosities of the brain (tig. 240) begin to be traced upon the hemispheres. The middle cerebral cell, which at the commencement was largest, increases less rapidly than the others. It is from this that the quadrigeminal bodies and the Sylvian aqueduct are evolved. The longitudinal fissure (322) is not distinctly trace- able till the fifth or sixth month. 1029. Medulla and Cerebellum. — The third or posterior cerebral cell (b) produces in the progress of its development the pons Varolii (fig. 240;), the medulla oblongata (fig. 240'), and the cerebellum. Two plates are produced, one at either side, which, bending towards each other, unite so as to assume the form of a nervous bridge over the ventricle of the cerebellum. The foliations of the cerebellum (fig. 2441’) are not developed until towards the close of the embryonic period. 1030. Fia and Dura mater. — The membranous coats which cover the spinal cord and the cerebrum, are developed contem- poraneously with the other part of the nervous system. Towards the third month they are perceived distinctly enough upon the encephalon. The pia mater is that which first appears, and can be distinguished easily at the end of the second month. 1031. The Spinal Nerves are developed in their natural position, and do not, as has been maintained by some anato- mists and physiologists, issue from the cord and the cerebrum by a sort of budding, like that of vegetables ; nor is it less in- correct to state that, when once formed, they direct themselves towards the cord and the cerebellum. They are formed, like the other parts of the nervous system in their natural position, from the general blastema, distributed between the cutaneous and mucous laminee of the germinal membrane, and the same is true of the great sympathetic system. 1032. The Eyes. — The organs of sense are severally deve- loped from the cellular structure of the encephalon. Each organ may he regarded as a mere expansion of the corre- sponding nerve. Thus, the two eyes result from the subdi- vision of a single cell, which is a sort of hollow emanation of the anterior cerebral cell (b, fig. 472). When the two ocular DEVELOPMENT OP THE ENCEPHALON AND COllD. 641 cells are once formed, tlieir anterior parts being of nervous structure, are reflected inwards upon the eye, and it is from these, spread over the interior surface of the organ, that the retina is formed. The other parts of the eye, within and without, are developed in like manner from the surrounding blastema. In this way the sclerotica and the cornea, the choroid, the iris, and the transparent humours are severally formed. The choroid is at first continuous, the iris not being yet per- forated by the pupil. It is not till the seventh month that the central part of the iris disappears, leaving open the aper- ture, called the pupil. Till the beginning of the third month the eyes are entirely covered by the skin, which, from this time becoming thinner and thinner, assumes the character of the conjunctiva. About the same time, the upper aud lower eyelids are developed in the form of cutaneous bands, which go on increasing until the fourth month, when they cover the whole globe of the eye. 1033. The Olfactory Organ consists of an excrescence of the anterior cerebral cell (b, fig. 472), forming the olfactory nerve, and its bulb, which in the beginning is hollow. The mucous membrane of the nose proceeds from the cutaneous lamina — -a part of which is enclosed by the development of the facial bones. 1034. The Organ of Hearing proceeds from the third or posterior cerebral cell (b). This cell, when developed, forms the nervous and membranous parts of the internal ear, and around it the bony parts are developed. About the third month the semicircular canals and cochlea, and also the external auditory meatus, and the concha are distinguishable. The meatus and the cavity of the tympanum are formed by the increase and development of different parts of the face. 1035. Vertebrae and Thorax. — Soon after the first appearance of the. spinal groove (fig. 471), there appears at either side of it a series of small quadrilateral plates arranged closely together, which, being cemented to the middle and anterior part of the spinal cord, form the bodies of vertebrae. A little later the vertebral plates in the posterior part of the blastema ore formed. They soon unite along the median line and upon the sides of the bodies of the vertebrae, and thus the 642 ANIMAL PHYSICS. vertebral canal is completed. The ribs and the sternum appear later. When the abdominal and thoracic cavities are formed by the curvature of the sides of the embryo, and the umbilical opening is clearly traced, the costal lines and the sternum are perceived in progress of development in the blastema interposed between the cutaneous and mucous laminae of the germinal membrane. It is about the sixth month of the embryonic life that the ribs and sternum are formed. 1036. The Skull is nothing more than a development of the superior vertebrae of the column. At a certain period its first vestiges can be traced in correspondence with the three principal divisions (b), which are severally designated the anterior sphenoidal vertebra, the posterior sphenoidal vertebra, and the occipital vertebra. The several bones of the skull are then formed progressively, and remain either in the state of dis- tinct bones or are cemented to those previously formed. 1037. Neck and Trunk — Mouth, Nose, Ears. — The different parts of the neck and trunk, are like the other parts, developed in the blastema, included between the lamina; of the germinal mem- brane. While the sides of the embryo are curved towards the centre of the egg, forming continued plates, and surrounding the abdominal and thoracic cavity, the parts which correspond to the neck and face consist not of plates but of tubercles, three at each side, which, being developed, form between them three openings. The tubercles take the form of three arches, called branchial arches, the development of which forms the jaws, with the teeth, the tongue, the soft parts of the face, the ossicles of the ear, the hyoid bone, the larynx, and the soft parts of the neck. The natural cavities of the face are formed by the parts of the original cavity of the embryo, bounded by parts developed from the tubercles. The first branchial cavity forms a sort of cloaca, common to the mouth and the nasal cavi- ties, but which is soon afterwards limited and shaped by the development of the maxillary bones and the nasal septum. The cavity of the tympanum of the ear, and the external auditory meatus proceed from the second opening, shaped by the develop- ment of the tubercles. 1038. The Limbs.— The pelvis, like the other parts, is formed from the interposed blastema. The limbs are deve- loped early ; and begin to make their appearance at the end of FORMATION OF THE ORGANS. 643 the first month. They appear, at first, nncler the form of small stumps on each side of the trunk, haying flattened extremities which constitute the rudiments of the future feet and hands. About the sixth week, the limbs having increased in length, the flattened terminal parts present four grooves, or incisions, which indicate the separation of the fingers and toes. 1039. Ossification commences in the bones at an early period ; but during the greater part of embryonic existence, they are more or less cartilaginous, and do not acquire their full hardness and strength until long after birth. 1040. The Muscles are traced in the blastema of the trunk and members, in their natural positions, about the end of the second month. The first seen are those of the vertebral column, a little later those of the neck, then those of the abdo- men, and finally those of the members and face. 1041. The Skin is formed from the cutaneous lamina of the germinal membrane, which covers the whole external surface of the embryo. This lamina may indeed be regarded as the primordial skin. As early as the second month the flattened and polygonal cells of the epidermis are distinguishable. In the third month the sudorific and other glands, as well as the rudi- ments of the nails, appear. The papillse of the skin are distin- guished about the fourth month. The hair is seen about the same period, under the form of a downy wool, and the eye- brows and eyelashes are developed about the sixth month. 1042. The Digestive Canal has its origin in the cavity of the boat-shaped body of the embryo already described. The intestinal sac enclosed within this first appears closed both at the cephalic and caudal extremities, communicating with the umbilical vesicle and allantois by the narrow ducts already described. Its communication with the umbilical vesicle corresponds with a point near the termination of the small intestine, and its connection with the allantois with the anal part of the intestine. These communications with the umbilical vesicle and allantois are soon obliterated, and the intestine becomes for the moment a tube closed at both ends. Being at first straight it soon takes the looped form, and is maintained posteriorly by a fold of new formation, which subsequently becomes the mesentery. The cephalic extremity of this closed tube, being enlarged, forms the. stomach. t t 2 644 ANIMAL PHYSICS. 1043. The Mucous Membrane which lines the intestine i> nothing more than the mucous lamina of the germinal mem- brane, somewhat modified in its structure. At its surface appears a cylindrical epithelium, and within its thickness villosities and glands. The muscles which line the digestive tube, the serous membrane which covers both it and the abdominal cavity now formed, proceed from the blastema accumulated between the germinal laminae. 1044. The (Esophagus is formed from the same source ; it soon opens at one extremity into the stomach, and at the other into the mouth. At its inferior extremity the digestive tube is placed in connection with a depression in the cutaneous envelope, called the rectal depression, and the septum by which they are separated soon disappearing, the entire intestinal canal is opened as well at its inferior as at its superior extremity. 1045. The Liver and the Pancreas are developed in the blastema beside the digestive canal. In the glandulous mass of the liver the excretory duct is seen developed, opening into the intestinal canal. The trachea and the lungs also make their ap- pearance, but separately, in the interposed blastema; the com- munications with the pharynx are, however, soon formed. 1046. Vascular System. — While the organs and tissues of the embryo are thus developed, the vascular system is formed, its first vestiges appearing nearly as soon as the spinal cord. These, like the other parts of the body, are formed out of the cellular mass of blastema, evolved between the lamina? of the germinal membrane. The first rudiments of the circulatory apparatus consist of blood-vessels which overspread the mucous lamina, and form upon it a circle nearly complete, called the terminal sinus, from which ramifications issue, communicating with the body of the embryo and others which cover the entire extent of that part of the germinal membrane which is soon converted into the umbilical vesicle. These vessels are placed in relation with the heart of the embryo, simultaneously developed near its cephalic extremity. 1047. The Primitive Embryonic Circulation may not inaptly be denominated umbilico-vesicular, since its purpose seems to be to supply nutrition to the embryo, by a, system of DIGESTIVE .CANAL — VASCULAR SYSTEM. C45 vessels which overspread the entire surface of the umbilical vesicle, and anastomose with the vessels of the villosity of the chorion. The vessels which issue from the superior part of the rudimentary heart are called aortic arches, and are curved downwards, being applied along the direction of the vertebral column. They separate at the umbilical opening into two arterial trunks, the ramifications of which .overspread the umbilical vesicle. These, called omphalo-mesenteric arteries, throw out ramifications which extend to the terminal sinus. From this a system of veins issue, called omphalo-mesenteric veins, which ultimately unite in two trunks, entering the body of the embiyo. The blood, issuing from the heart through the omphalo-mesenteric arteries, is carried by them to the terminal sinus, and brought back by the omphalo-mesenteric veins. This first embryonic circulation will be rendered more intelligible by the theoretical diagram, fig. 473. Fig. 478. PRIMITIVE EMBRYONIC CIRCULATION. a, a. Vitellus or yolk. b, The heart. c, c. Amnion. /, /. Terminal sinus. d, d, d. Omphalo-mesenteric veins. g, g. Omphalo-mesenteric arteries. The aorta divides into two omphalo-mesenteric arteries, (jrj right and left, which ramify over the germinal membrane until they reach the terminal sinus/. The blood is brought back by the mesenteric veins which coalesce in the trunks d d. Subsequently, other veins are developed in the vascular network, and, at length, when the germinal membrane has enclosed the whole yolk, the terminal sinus entirely disappears, and the yolk sae becomes covered with blood-vessels. 646 ANIMAL PHYSICS. As a further illustration of this primitive embryonic circulation, we give that of the dog in the 23rd or 24th day of the embryonic period, magnified 10 diameters, in fig. 474. Fig. 474. PRIMITIVE CIRCULATION OF THE DOG (Bisclioff). This figure shows the network of blood-vessels in the vascular lamina of the germinal membiane, and the trunks of the omphalo-mesenteric veins entering the lower part of the S-shaped heart. The first part of the a rta is also seen. The vessels which thus circulate over the umbilical vesicle receive, by absorption, the liquid matter contained in this vesicle, and cam- these materials to the embryo by the omphalo-mesenteric veins. The umbilical vesicle and its vessels thus play, to a certain extent, the part with rela- tion to the embryo which is afterwards performed by the placenta. 1048. The Second Embryonic Circulation commences when the communication between the intestine and the umbilical vesicle disappears. The omphalo-mesenteric vessels, reduced at first to a single artery and a single vein, atrophise, and finally disappear with the vesicle itself. The intra-embryonic portion of the omphalo-mesenteric vein continues to receive the venous blood of the intestines by the mesenteric vein, which latter forms the trunk of the portal vein. As a prelimi- PRIMITIVE EMBRYONIC CIRCULATION. 647 naiy to the establishment of the second circulation, the allantois increases with the decrease of the umbilical vesicle. a, b. c. The chorion produced by the incorporation of the vitelline mem- brane, the cutaneous lamina of the germinal membrane, and the mem- brane of the allantoid vessel. See also fig. 470. c. The umbilical vesicle in progress of decrease. d. The cephalic, and d' tho caudal extremity of the embryo. e. The ventricular, aud /, the auricu- lar cavity of the heart. i. The aorta formiug the arcs. h. The thoracic aorta. g. The trunk which ultimately forms the vena cava superior. k. Tho trunk of the azygos vein. l. The confluence of g and k. m. The general confluence of all the veins on entering the auricular cavity. n. Tho trunk resulting from the union of the allautoidal p p and om- phalo-mesenteric vein q. o. Vena cava inferior. p. p. Allautoidal veins. q. Omphalo-mesenteric vein. r. Abdominal aorta. *, a. Allautoidal arteries. t. Omphalo-mesenteric artory. No sooner has the allantois appeared than its surface is 648 ANIMAL PHYSICS. covered with vascular ramifications which extend to the internal surface of the chorion, with vascular ramifications of the villositv of which its vessels .anastomose at the periphery. The vascular communication between the maternal and the embryonic oiganism is thus established. About the end of the first month, there is a moment when the embryonic circulation includes at ouce the principles of the primitive and the second circulation, the umbilical having not yet quite disappeared while the allan- toidal is being established. This intermediate stage is illus- trated in fig. 475. The vessels of the allantois are at first four in number, two arteries, s, t. and two veins, p, p ■ but when this vesicle has played its part in the embryonic phenomena, one of the veins atrophises, the two arteries and the remaining vein continuing till the transition from embryonic to natural life, form, at the moment of the transition, the vessels of the umbilical cold. The two arteries then communicate respectively with the iliac branches of the descending aorta, and the vein with the portal vein and the inferior vena cava. 1049. This second vascular system is complete about the commencement of the third month, and its functions are maintained unaltered till the close of the embryonic period. The manner in which the circulation takes place is as follows: — The heart in its rudimentary state has the form of an elongated sac, lying at the fore-part of the embryo, and having two veins connected with it behind, and a large arterial trunk in front. It soon becomes bent upon itself into the form of a horse-shoe, and as this curvature increases, three enlargements, or cavities, are formed, the first corresponding to the auricle, and called the auricular , the second to the ventricle, called the re utricular. and the third to the origin of the aorta, and called the arterial bulb. This latter cavity ultimately becomes divided into the origin of the pulmonarv artery and aorta. A septum between the two ventricles is soon formed, and one between the auricles commences, and about the fourth month of the embryonic period, increases to a certain point, leaving, however, an opening called the foramen ovale, or hole of Botal, which maintains a communi- hication between the two auricles, until the close of the embryonic period. This constitutes the main difference between the structure of the embryonic and that of the natural heart. The structure of the embryonic heart will be rendered more easily intel- ligible by fig. 476, where A exhibits a front, and b a back view of the heart, at about the fifth week of the embryonic period. In the progressive development of the vascular system, the aortic arches proceeding towards the cephalic extremity are multiplied and form a cer- tain number of secondary arches, which correspond to the formative tuber- cles of the face and neck (1040) ; these arches, being modified, produce the cross of the aorta, the pulmonary, subclavian, and carotid arteries, and their branches. It is important to observe, however, that a large com- munication, called the arterial duct, exists during the embryonic period SECOND EMBRYONIC CIRCULATION. C49 between the aorta and the pulmonary artery, which is not obliterated till the transition takes place from embryonic to natural life. A A. Tlie heart seen on the ventral aspect and laid open. 1. Arterial bulb. 2. 2. Arterial arches which unite be- hind the aorta. 3. Auricular region. 4. Opening leading from auricular, 3 , to ventricular cavity, 0. 5. Septum of the ventricles com- mencing to be formed on the floor of the cavity. 7. Vena cava inferior passing through the diaphragm. B. Back view, with the rudiments of the respiratory organs. 1. Larynx and trachea. 2. Lungs. 3. Ventricular, and 4. Auricular part of the heart. 6. Diaphragm. 7. Descending aorta formed by the union of the right and left aortic arch. 5. 9, 10. Trunk and branches of pneumo-gastric nerves. All the circumstances attending this second embryonic circulation, which continues to the close of the embryonic period, are illustrated in fig. 477. The arrows indicate the course of the blood along the several blood- vessels. The blood of the superior cava c* mostly descends tlirodgh the right auricle r, as shown by one arrow, into the right ventricle v*. That of the inferior cava c, ascends through the right auricle r, passes, as shown by another arrow, into the left auricle l, and thence into the left ventricle v. The course of the blood from the right ventricle into the pulmonary artery/), and ductus arteriosus o, and from the left ventricle into the aorta, », is shown by two dotted lines. The right auricle r, receives its blood from the two vense cava; c and c*. The blood brought by the superior cava c* is the venous blood which returns from the head and upper part of the body. The inferior vena cava, which is considerably larger, conveys the blood from the lower half of the body, and also that which is sent back from the placenta, through the umbilical vein u. This umbilical vein terminates in the figure before its junction with the placenta. The blood which passes through the umbilical vein v, has been arterialised in the placenta by endosinose with the maternal organism. This blood passes into the inferior cava c, in two different ways, first by the ductus venosus d, and, secondly, by the hepatic veins h, which receive it after having passed through the liver, to which it was conducted by the portal vein g. The blood supplied by the superior cava c*, descending over a valve called the Eustachian valve, and mixed with a small portion of that which 650 ANIMAL PHYSICS. comes from the inferior vena cava, passes into the right ventricle v*, and JLLUSrRATION OF THE ANTE-NATAL EMBRYONIC CIRCU- LATION (Sharpey). * *. The aortic arc and descending aorta; i i, the umbilical arteries pro- ceeding from the iliac ar- teries, which diverge irom the lowest point of the trunk t, and proceed at a, to the umbilical cord winding spirally round the umbilical vein. u, the continuation of which (not represented in the figure) goes to the embryonic placenta iu wliich the arteries and vein ramify. u. The umbilical vein. d. The ductus venosus, a branch of the umbilical vein which goes direct to the inferior cava, c. but which ceases to exist after birth. *. The branch of the umbilical vein which joins the vena portae. <7. The vena portae, which returns the blood from the digestive or- gans. lu The hepatic veins, which return the mixed portal and placental blood after its circulation through the liver. c. The inferior cava. c*. The superior cava. r. Right auricle. v*. Right ventricle. L Left auricle. v. Left ventricle. p. Pulmonary artery. o. Ductus arteriosus, connecting p with the aorta, which ceases to exist after birth. s. Arch of the aorta. is thence propelled to the pulmonary artery p. A small portion of it is then distributed through the ramifications of that vessel to the lungs, and returns by the pulmonary veins to the left auricle, but by far the larger part passes through the arterial duct o, into the aorta .«, below the origin of the arteries of the head and upper members. This is probably mixed with a small quantity of blood flowing along the aorta, from the left ventricle, and descends, partly to supply the lower half of the body and the viscera, hut chiefly to be conveyed along the umbilical arteries i >, to the placenta. From all these parts it is sent back by the inferior cava, the veua portal, and the umbilical vein to the right auricle. ANTE-NATAL CIRCULATION. Go l The blood' of the inferior cava is partly distributed, as already stated, with that of the superior, but the larger portion directed by the Eustachian valve through the foramen ovale flows from the right r to the left auricle I, and thence together with the comparatively small quantity of blood returned from the lungs by the pulmonary veins, passes into the left ventricle v, from whence it is sent into the arch of the aorta s, to be distributed almost entirely to the head and upper members. A certain part of it, however, flows most probably into the descending aorta joining the large stream of blood from the arterial duct. The blood from the upper half of the body is returned by the branches of the superior cava to the right auricle from which its. course has been already traced. It will appear from what has been stated, that the chief part of the arterialised placental blood goes to the cephalic half of the embryo, the lower half beiug chiefly supplied with blood which has already circulated through the head and limbs. The analogy of this part of the circulating system of the embryo to that of amphibia and certain reptiles will be obvious. 1050. Lower Mammifers. — The phenomena of embryonic life, explained in the preceding paragraphs, are common (with certain modifications in the marsupial and monotrematous orders) to all mammifers. This class of animals, as their name implies, nourish their young, in the first period of post-natal life, by milk, secreted in the breasts or paps. The principal differences in the phenomena of reproduction depend upon the number of young produced at a birth, the duration of embryonic life, and the periods of breeding, which are shown in different species in the following table : — No. in Brood. Embryo period weeks. Cow 1 41 Horse 1 43 Staff 1 36 Camel 1 45 Elephant 1 100 Ass 1 43 Ape 1 Bear 2 30 Deer 2 24 Bat 2 Hare 3 to 4 4 Beaver 3 to 4 17 Mole 3 to 4 Marmot 3 to 4 5 Guinea pi z .... 3 to 4 3 Lion 4 to 5 14 Tiger 4 to 6 Leopard 4 to 5 No. in Brood. Embryo period weeks. Dog 5 to 6 9 to 10 Fox 5 to 0 9 Wolf 5 to 0 10 Cat 6 to 6 8 Weasel 5 to (5 5 Squirrel 5 to 0 4 Rabbit 0 to 8 4 Water-rat G to 8 Field-mouse G to 8 Fen-et 0 to 8 G Mouse 8 to 10 3 Pig 12 to 15 17 Rat 12 to 15 5 Polecat » Sheep 21 Goat 22 Zebra 43 1051. Number of Young in a Brood.— When animals habi- tually produce several younglings at a brood, the number G52 ANIMAL PHYSICS. is subject to more or less variation, and those given in the above table are consequently to be taken as averages. The less the number produced, the less will be the variation ; and in animals which produce habitually only a single youngling, the cases of two or more at a birth are always extremely rare and exceptional. 1052. Multiple Births in Man.— In general, man comes into the world singly, or one at a birth. The cases of twins are rare, and those in which three or more at a birth are produced extremely exceptional. It appears, by statistical returns made upon a large scale, that upon an average one case of twins occurs in ninety births, and that of three or more at a birth has not occurred more than once in thirty thousand cases. Rare, nevertheless, as such multiple births are, cases are recorded in which they have occurred with surprising frequency in single families. Thus, a Russian peasant was presented to the Empress Catharine as a natural curiosity, having been the father of ninety children. It is true that this numerous family had not a common mother ; and the most singular circumstance attending the case was, that in several successive marriages the births had been invariably multiple. 1053. Hypothesis to explain them. — Physiologists have endeavoured to explain the phenomena of exceptional multiple births by the organic accident of two or more Graafian follicles attaining maturity or bursting and discharging their ova at the same time, or nearly so. The ova thus discharged produce embiyos which are simultaneously developed. Another hypo- thesis is, that two or more ova may happen abnormally to be contained in the same Graafian vesicle. These hypotheses, how- ever, must be taken for what they are worth, being scarcely capable of verification by observation. 1054. Proportion of the Sexes. — The human race is distin- guished from inferior animals by the independence of the phenomenon of birth on the season of the year. Animals generally produce their young at that season which is most favourable for their development. Children are born at all seasons. Nevertheless, the frequency of births has a marked and well ascertained relation to the course of the seasons. It is found generally in the temperate climates, that births are most numerous in the three winter, and least so in the three summer months. In approaching the colder climates, the NUMBER OF YOUNG. EMBRYrONIC PERIOD, 653 epochs of the maximum and minimum numbers are later, and in approaching the warmer climates earlier-. The number of children which come into the world is not equally shared between the sexes, the male always predominating. This fact has been established in all countries where statistical registers have been kept ; and it is remarkable, that although the numerical proportion between the sexes is subject to some variation from year to year, its mean amount in each country is nearly invariable, though different in one country as compared with another. Thus, on comparing the numbers of male and female children baptised in England and Wales during the first half of the present century, it is found that the number of males invariably exceeded the number of females in a proportion varying, from year to year, from 25 to 50 per 1000 ; the mean result taken for the whole period showing, that for every thousand girls bom, there were one thousand and forty boys. In France, according to returns extended over 36 years, terminating in 1852, it appears, that for every thousand girls there were one thousand and sixty-one boys born. Thus the preponderance of male births in France exceeds that in England in the proportion of a little more than 6 to 4. By returns obtained from other countries where accurate statistics are kept, it has been found that the preponderance of male births Is intermediate between those of England and France, the number of males being 1050 for every 1000 females. 1055. Birds. — The oviparous classes offer great facilities to the researches of the embryologist, in consequence of the ejection of the egg at a very early epoch in the progress of the develop * inent of the embryo ; and in the class of birds especially these facilities are materially increased by the circumstance of the egg being so easily and conveniently submitted to artificial incubation. It will not therefore be surprising that birds, certain oviparous amphibia, such as frogs and salamanders, and certain reptiles, such as some species of serpents, have been severally, but more especially birds, subjects of the most important and successful researches in this branch of physiology. Before stating the principal results of these researches directed to the class of birds, it will not be without interest to explain the measures which nature has prompted these animals to adopt, preparatory to the deposition of the egg, for the more effectual' preservation of their species — measures which, if not ascribed 654 ANIMAL PHYSICS. to Heaven-sent instinct, would manifest a degree of skill, fore- sight, and parental solicitude which would do honour to the rnost intelligent and refined of the human race. 1056. Nests. — As the laying season approaches, the bird, though it were conscious of the coming event, occupies herself in the construction of a dwelling, suited by its materials and form to the little beings to which she is about to give life. Such a structure must fulfil several conditions. In its magni- tude and form it must correspond with the magnitude and number of the eggs to be laid, and with the body of the mother who is to sit upon them. It must be so shaped as to keep the eggs securely together, and its materials must be soft, so as not to injure by undue pressure its tender occupants. To prevent the escape of the warmth imparted by the mother, it must be thickly lined with non-conductors of heat. All these conditions are fulfilled with the skill of a natural philosopher. The nests of the larger class of birds, of hardier nature, are of rude construction ; but those of the smaller species display in a remarkable manner the qualities here indicated. The parents of the coming oflspring, father and mother, co-operate in the construction of the nest, for the external part of which straws and twigs are collected, and woven into a sort of basket-work. This not possessing the requisite durability, and allowing, more- over, the air to penetrate and the heat to escape, a quantity of fine clay is collected with enormous labour, and worked into a sort of mastic by means of a viscous fluid secreted by glands placed under the tongue of the bird. With this mastic the parents plaster the interior of the nest, carefully stopping up every crevice and air-hole. The floor of the nest, however, formed by such plaster, is necessarily hard, and would injure the younglings by its pressure. The parents, therefore, fabricate a carpet, which they spread upon it, over which the}7 place a soft mattrass, the materials of which consist of wool and fine hairs taken from the backs of animals and the cottony parts of certain plants. How countless are the excursions and fatiguing the labour necessary to accumulate, hair by Jiair and filament by filament, such materials, may be easily conceived. Sometimes the bird strips its own breast of its natural down to form a bed for its young. It is thus that the eider duck provides for the comfort of its oflspring, by taking from its own body part of that down which is sought and collected at such cost for the pillow of luxury. DEVELOPMENT OF BIRDS. 655 1057. The Laying heaving commenced, and the eggs having been deposited, the mother sits upon them with a constancy so unwearied, that it often affects her health. In some species the two parents share this labour, each allowing the other intervals for refreshing exercise. In other species, while the mother sits upon her brood, the father constantly flies to and fro, bringing nourishment to her, which he presents in his bill with the greatest apparent tenderness. The male of singing birds also often displays his conjugal affection in a still more remarkable manner, by singing beside the nest to wile away the weary hours of the mother of his future offspring. 1058. The Brood. — When the young have broken the shell and issued into external life, a new sphere of action succeeds, The parents are now incessantly occupied in providing the family with food, which they swallow in a raw state, taking it into their crop or first stomach, an organ which has been ak-eady described (729). There it undergoes a change which renders it suitable to the digestive functions of the young birds, into whose open beaks they disgorge it. As the younglings acquke strength, the parents often devote themselves to thek education. They encourage thek first efforts at flight, and teach them where and how to find food. In the case of bkds of prey, the parent actually gives the young a series of progressive lessons in the manoeuvres necessary for the seizure of its prey.* 1059. The Embryonic Development of the Bird consists of three distinct periods : the period of the ovary, the period of the oviduct, and the period of the incubation. In the first the germ is produced and invested with the germinal vesicle, the yolk, and the yolk sac. The theatre of these phenomena is the ovary. In the second, the egg is rendered capable of embryonic development, and is invested with the white or albumen, the shell and certain membranes inclosing these. The theatre of these phenomena is the oviduct. In the third, the embryo is organised from the materials included in the shell ; and at its close the young bkd, breaking the shell, issues into external life. The phenomena of this period are produced by the warmth of the maternal body in the * Lardncr’s Museum of Science aud Art, vol. VIM. p. 100. ANIMAL PHYSICS. (15 G act of incubation, and by the action of the atmosphere in contact with the shell. 1060. The Ovary (fig. 478), like that of mammifers (fig. 4G0), consists of an assemblage of ovicap-. sules, called also ovisacs or calyces, in each of which an egg is produced. When the egg has arrived at a cer- tain stage of development, being con- siderably augmented in magnitude, the calyx bursts, and the egg is ex- pelled and enters the superior end of the oviduct. The calyces composing the ovary are in various stages of development, a few being nearly ripe, but the greater number minute and in a comparatively backward state. Their dehiscence and the expulsion of the eggs therefore takes place one by one, in regular succession, a certain interval intervening between the escape of the eggs. 1061. The Calyx is a hollow spheroidal case surrounded by several layers of blood-vessels, lymphatics, and nerves, connected together by cellular tissue and peritoneum. The blood-vessels on the external surface are composed of two layers, the exterior consisting of vessels of large calibre, and the interior of a much smaller reticulation. The inner or concave surface is tufted over with a vascular villosity, in which the germ of the egg is produced by the process of secretion, and in which the veins predominate greatly over the arteries as well in magnitude as in number. Fig. 47S. bird’s ovary (Carus.) Fig. 479. As the calyx enlarges, its vascular activity undergoes a rapid increase, all the vital energy of the ovary ap- pearing to be, for the moment, con- centrated in those calyces which are about to deliver their eggs to the oviduct, those in the more backward state being temporarily inert. This concentration of the formative activity, producing gradations of development in the egg, is in accordance with the functions of the animal. If several calyces ripened and enlarged together, the ovary would attain undue and injurious magnitude, and the eggs would be discharged into the oviduct CALYX FULLY DRVF.I.OPED, SHOWING THE . , • . suc0essj011 that that SURROUNDING BLOOD-VESSELS AND THE 1,1 S,,C11 , S'lCCeSSlim, TDai lUSl stigma (Baudrimout and Martin St. organ would be incapable ot secreting Ange). the accessory parts which it is its function to supply. The secreting powers therefore of the ovary and 657 BIRD’S OVARY. — CALYCES. the oviduct are nicely adapted each to the other, egg after egg being evolved in the one at such intervals as enables the other to supply the albumen, the membranes, and the shell. When the calyx is in a state of full development and ready to expel the egg, the ovary and the numerous blood-vessels which interlace its sides acquire an enormous volume. A slightly magnified view of the calyx is shown in fig. 480, and one is represented in fig. 479 in its natural magnitude after the discharge of the egg. 1062. The Stigma. — Their ramifications surface, except a semicircular band extend- ing over that part along which the fissure is subsequently produced through which the egg is expelled. It will be seen by the figure that the limit of the visible blood-vessels is distinctly traced in two parallel directions along the edges of this zone, which is called the stigma. Until recently it was maintained by physiologists that this stigma is not vas- cular. Messrs. Baudrimont and Martin Saint- Ange,* however, have shown that this is not strictly true, for although the stigma is divested of the larger class of blood-vessels, it is nevertheless the seat of minute capillaries. 1063. The Egg, when first formed in the ovary, appears under the form of a minute semi-transparent vesicle, proved by these physiologists to be produced, not as hitherto supposed by a sort of budding or exfoliation, but by a process of true secretion. As it enlarges within the calyx, the vitellus or yolk collects round it, enclosed by the vitelline membrane. In the midst of the yolk the germinal vesicle, including the germ, is formed. The germinal vesicle is placed in the centre of the whitish spot which constitutes the cicatricula. When dismissed from the calyx, and discharged into the oviduct, the egg, therefore, consists of the vitelline membrane, enclosing the yolk, within which is the germinal vesicle, enclosing within it the proliferous disc and the germ. 1064. Chalazae. — On entering the oviduct, the egg thus constituted becomes fecundated and encounters the albumen in * Mem. Acad. 8c. Tom. XI. cover the entire Fig. 480. n u 658 ANIMAL PHYSICS. abundant quantity secreted by that tube. This collecting round the yolk, is condensed upon it, and formed into a layer, which applies itself in immediate contact with the vitelline mem- brane. The egg, as it advances through the oviduct, is endowed with a rotatory motion around its axis, by means of which the albuminous envelope just described is twisted at both ends, just as a towel would be when water is wrung from it. By these means, those twisted parts of the albuminous coat which extend beyond the vitelline mass, form those appendages called the clialazce. The albuminous membrane here described has on this account received the name of the membrane of the chalazee. 1065. Membrane of Albumen. — In its further progress, the egg collecting round it an increased quantity of albumen, that substance becomes condensed upon it in contact with the membrane of the chalazse, and another membrane is formed on the exterior of this condensed albumen, consisting of a fine transparent pellicle, called the membrane of the albumen. 1066. Membranes of the Air-Chamber. — As it continues its further progress through the oviduct, it receives a fresh accession of albumen, which is now fluid and less condensed. Upon this fluid albumen is formed a double membrane, called, for a reason which will presently appear, the internal and ex- ternal membranes of the air-chamber. These membranes, there- fore, contain, in a closed sac, all the previously formed parts of the egg. 1061. Isthmus. — The egg in this state arrives at a point of the oviduct called the isthmus, where the secreting organ of the albumen terminates ; after passing which, it enters a lower part of the oviduct, in which a calcareous secretion takes place, by ’ which the materials of the shell are deposited upon the external membrane of the air-chamber just described. This deposit immediately solidifies, and the shell is constituted. 1068. Epidermoid Membrane Then comes a last secre- tion of another order, by which a superficial membrane, called, from its analogy to the epidermis, the epidermoid membrane, is formed upon the external surface of the shell. The material of this last secretion contains a colouring matter which ■varies with the species of the bird, and gives to the egg its charac- teristic colour, whether uniform, varied, or spotted. PROCESS OF OVIPOSITION. 659 1069. Theoretical Section of Egg. — Tlxe constitution of the egg here described will be rendered more clearly intelligible by the theoretical section (fig. 481), where the parts above described are indicated as follows : 3 2 1 Fig. 481. theoretical sectiok of the hex's eco (Baudrimont and Martin St. Auge). 1. Epidermoid membrane of the shell. 2. The shell. 3. External membrane of the air chamber, applied immediately upon the internal surfaco of the sholl. 4, 4. The internal membrane of the air chambor applied upon the inner surface of the preceding membrane everywhere except at the large end of the egg. 5. Tho air-chamber. 6, 6, 6. The membrane of tho condensed albumen. Between this membrane and the internal membrane of the air-chamber tho liquid albumen is included 7. The membrane of the chalaxie. Between this and 6, 0, 6, the con- densed albumen is included. 8, 8. The chalazse twisted at tho ends, as already described. 9. The vitelline mombrane. 10. Tho germinal membrane, bUiModenna or cicdlricula, a small flat disk in which the rudiments of tho ombryo afterwards begin, and which probably receives the contents of the germinal vesicle, now ruptured. 1070. While the formation of the accessories of the egg is u u 2 660 ANIMAL PHYSICS. in progress in the oviduct, its essential parts are modified. The germinal vesicle visible at first has disappeared, being broken, and its contents diffused in the concavity of the blastoclerma, or germinal membrane or disc, fig. 481 10 . At this point the organs of the chick are represented only by their materials collected together in a shapeless condition. Nothing more, however, is necessary to awaken the functions of assimi- lation than a suitable temperature and a sufficient supply of oxygen to favour sanguification. 1071. Air-Chamber. — The first of these conditions is ful- filled by the abdomen of the mother, and the other by the air- chamber (481 5), a cavity produced by the partial evaporation of the water contained in the albumen, and its replacement by air passing through the pores of the shell. This chamber, which always occupies the large end of the egg, is limited by the two membranes which take their names from it, the exterior of which is applied against the shell and the interior (4 8 14) upon the albumen. 1072. The Process of Embryonic Development com- mences from the first moment of incubation. About the Fig. 4S2. TRANSPARENT AND VASCULAR AREA ABOUT THE 37tH HOUR OF INCUBATION (Baudrimout and Martin St. Angc). sixth hour the blastoderma or germinal disc is raised above the mass of tho yolk. Tluee hours later it becomes opaque. SECTION OF THE EGG. G61 and its volume rapidly increases. Having at first no more than the eighth of an inch in diameter, it acquires at the end of twelve hours a diameter of half an inch. Fecundation having impressed a vital movement on the globules or cells accumulated in the disc, these arrange them- selves in such a manner as to foim two distinct regions, one peripheric, and called the opaque area, and the other diametral, and named the transparent area (fig. 482). The transparent area is represented here lying along the diameter of the vascular area. Along the centre of the transparent area appears the double encephalic cord, already flanked by the rudiments of the vertebra;, and upon the sides a multitude of fine vitelline granulations are collected, which are crowded one against the other, prepared to undergo the metamorphoses destined to convert them into the organs of the young animal. The globules of the blood put in motion by the cardiac vessel mark out the vascular area pressing aside the fatty vesicles contained between the lamina; of the blastoderm. It is not until a later epoch of the develop- ment that the motion of these globules is limited by vascular tubes. 1073. Vertebral Column. — At the beginning of the second day of incubation part of the formative molecules group themselves to organise the vertebral column, and two parallel rows of square spots are formed, separated one from the other by a pellucid line. These are destined ulti- mately to unite, two and two, on the internal sides, to form the bodies of the vertebrae. The line separating the two rows is occupied by a transparent fluid which, being gradually condensed, forms the spinal marrow and the brain. 1074. Skull and Organs of Sense. — The cerebro-spinal axis is soon after bent, the cephalic extremity inclining forward, and it is at that moment that on both sides of the skull the cavities of the orbits are formed, in which are subsequently accumulated the humours destined to form the organs of vision. Next appear at the posterior part two other depressions prepared to receive the rudiments of the organs of hearing. 1075. Bones — Muscles — Members. — The first formation of the bones is announced by the appearance of cartilages destined eventually to become hardened by calcareous matter. The muscles are at first developed in the dorsal region ; after which two pairs of stumps make their appear- ance, one at the superior, and the other at the inferior part of the embryo. These being gradually elongated and subdivided by articulations, are ulti- mately converted into the superior and inferior members of the animal. 1076. Vascular Apparatus.— At the commencement of the third day the vascular system has not yet been manifested. The whole surface of the transparent area (tig. 482) appears granular. These granulations represent innumerable transparent globules, which also abound in the opaque area, where they are mixed with oily vesicles. These globules and oily vesicles are enclosed in a double lamina, within which the blood is formed. The blood appears first in the form of spherical corpuscles CG2 ANIMAL PHYSICS. slightly coloured and a little opaque at the centre. These soon after assume the forms of elliptical globules of a bright red colour, with a central nucleus. According as these blood corpuscles multiply, the oily vesicles and granules diminish in number, which leads to the probable supposition that the latter enter into the composition of the former. It is about the fiftieth hour of the incubation that the blood corpuscles become distinctly visible. If the disc be then observed, it will be found that all the globules are agitated by a regular motion, and seem to force their way among the oily vesicles. It is evident that the blood-vessels, properly so called,’ are not yet formed. The blood therefore, at first, makes for itself a path in the tissues, and it is not until a later period that the system of vascular canal - is formed, by which it is guided in its motion. 1077. The Circulation of the Embryo of the Chick, like that of the mammifer, undergoes a series of changes with the progress of organic development. These changes consist of four phases, which have been denominated severally the prirrdtiv circulation of the. vitellus, the permanent circulation of the vitelhts, the circulation of the allantois , and the definitive circulation of the chick. 1078. The Primitive Vitelline Circulation, the duration of which is two days, takes place in a part of the germinal mem- brane called the vascular area, which with the rudiments of the heart of the chick and its cardiac vessels in the centre, is represented in fig. 485, with a diameter magnified about six times. The arteries are distinguished from the veins by a lighter shading. 107fi. The large vessel v P, by which the vascular area is surrounded, is called the primigenial vein. Externally it has no branches, but is con- nected with the network within by innumerable ramifications. Its course around the area is somewhat irregular, throwing in at certain points re-entrant angles, and at others contracting its calibre, so as to form a comparatively narrow neck. At a point corresponding with the position of the cephalic extremity of the embryo it turns inwards with a deep and very acute re-entrant angle, at the vertex of which the converging points coalesce so as to form a single trunk, which is continued to the heart, which it enters at an enlargement of that organ, to which MM. Baudrimont and Martin St. Ange have given the name of subcardiac vessel. 1080. At a part of the vascular area diametrically opposite to that from which this trunk comes, another large venous trunk (v c), called the caudal vein, proceeds and enters the heart at the same point. Both of these trunks communicate with the surrounding vessels by numerous ramifications. Two other venous trunks of still larger calibre enter the heart at the sides, a fifth receives branches, proceeding from the body of the embryo. The motion of the blood in all these five trunks directed from the borders DEVELOPMENT OF THE CHICK. 663 of the area to the heart was distinctly seen by the observers already cited. V.P. Fig. 4S3. VASCULAR AREA, SHOWING THE PRIMITIVE CIRCULATION OF THE VITELLUS (Baudrimont and Martin St. Ange). The venous blood flowing from these trunks into the subcardiac vessel, which may be regarded as the analogue of the portal vein, is discharged into a curved vessel susceptible of a regular rythmical contraction, and which is the rudimentary heart. 1081. The heart which lias thus received the venous blood coming from the circumference to the centre pushes, in its turn, the blood through the arteries from the centre to the circumference. The vascular area, which is the theatre of this primitive circulation, may be regarded as a sort of lung formed above the embryo, and placed under the air-chamber, where the oxygen can enter it most easily. This constitutes a simple pulmonary circu- lation analogous to that of fishes, the duration of which is about two days. 1082. The primitive vitelline circulation described above is terminated by the ramifications of the arterial and venous vessels, which run parallel to each other between the centre of the vascular area and the primigenial vein, anastomosing ; in conse- quence of which the primigenial and caudal veins, receiving an insufficient supply of blood, soon atrophise and wither. 6G4 ANIMAL PHYSICS. 1083. The Second Circulation then commences. The vas- cular area, deprived of its principal venous trunks, and ceasing to communicate so fully as before with the air-chamber, becomes less fitted for respiration, but its nutritive functions, on the other hand, are improved. It spreads more and more over the yolk, whence, its vessels playing the part of chylifers, derive abundant materials for the increasing organisation of the embryo. 1084. Allantois. — Meanwhile the pulmonary functions are transferred to another and different apparatus. According as the vascular area loses its primigenial and caudal veins and their tributaries, the allantois appears, and is converted into a new respiratory organ, eminently suited by its con- nection with the embryo to favour the development of the interior parts of the animal, hitherto retarded by the peculiar nature of the primitive circulation. Before, however, describing this new phase of the circulation, it will be convenient to illus- trate the progressive development which has taken place during the first phase of the circulation. 1085. A section of the egg shown in fig. 484 is in a more advanced stage than that already shown in fig. 481. In this figure is seen one of the chalazse, 8, the other being hidden on the opposite side of the egg. 11' is the cicatricula upon which the vascular area is spread. 9' is the internal granular layer of the vitelline membrane which some physiologists have, erroneously, according to Messrs. Baudrimont and Martin St. Ange considered as constituting a part of the blastoderm, out of which the embryo is formed. In fig. 487 is represented a theoretical section of the egg, showing the embryo in a stage of still further development. In this case 11' is the "germ more developed, bulging up the vitelline membrane. 11, 11 is the primi- genial vein included between the two lamina- of the germinal membrane and extending itself more and more under the vitelline membrane in the place of the granular layer which lines the latter. The membrane of the chalazse is raised by the embryo as well as the vitelline membrane, of which, however, it does not exactly follow the contour. A section of the egg illustrating a still more advanced stage of the development of the embryo is shown in fig. 486. 11' is the embryo beginning to assume a distinct form. 1 1, 11 are the folds of the membrane about to form the cephalic and caudal hoods. About the end of the third day of the incubation the allantois begins to be developed from the abdominal region of the embryo. The state of the egg at this period is illustrated in fig. 487. The parts numbered from 1 to 10 are the same as those in fig. 4S1. One only of the cbol&zse (8) appears, the other being on the opposite side. 9, 9'. Internal granular layer of the vitelline membrane. 10. The embryo beginning to be developed and depressing the yolk. 11. The primigenial PRIMITIVE CIRCULATION OF THE CHICK. 665 vein, already atrophised, limiting the vascular area, which extends itself more and more in the place of the granular stratum of the vitellus. 13. Folds of the cephalic and caudal hoods approaching each other, the space Fig. 484. THEORETICAL SECTION OF THE HEN’S EOC IN THE SECOND STAOE OF DEVELOPMENT (Baudrimont and Martin St. Ange). between them being the umbilical opening of the amnion, which, however, very soon disappears when the development of the embryo progresses. 15, 15. The cavity of the amnion. 14. The pedicle of the allantois, corresponding to the cloaca. The allantois makes its appearance at the moment of the metamorphosis of the vascular area. It supplies the place of the vitelline organ whose respiratory function gradually ceases with the disappearance of the primigenial cephalic and caudal veins. The next stage of the development is shown in fig. 488. 1 to 9. The same parts as in the former figures. 10. The embryo in a more developed state. The primogenial vein no longer exists. The vitelline veins and artery have arrived at the limit of their development, and constitute the vitelline circle 16. 666 ANIMAL PHYSICS. 12, 12. Folds of the vitelline membrane which form the cephalic air; caudal hoods. Fig. 4S5. THEORETICAL SECTION OF THE HEN’S EGG IN THE THIRD STAGE OF DEVELOPMENT (Baudrimont aud Martin St. Ange). 15, 15. Cavity of the amnion, which is more and more enlarged by the gradual accumulation of the serous transparent fluid called amniotic liquid. 14. The allantois commencing its development and pushing before it the vitelline and chalazean membranes. 16. The vitelline circle. 1086. From the fifth to the sixth day the allantois covers the little embryo, aud circumscribes all the parts included in the internal membrane of the air-chamber. About the tenth day it has already surrounded and embraced the whole of the vitellus. the embryo, and the albumen. From the twelfth to the PROGRESSIVE DEVELOPMENT OF THE CHICK. 6C7 thirteenth day the junction of the allantois has taken place at the small end of the egg. Its external fold, which lies against Fig. 4S6. THEORETICAL SECTION OF HEN’S EOO IN THE FOURTH STAGE OF DEVELOPMENT (Baudrimont and Martin St. Ango). the internal membrane of the air-chamber until the chick breaks the shell, is covered with a magnificent vascular system, which receives the venous blood coming from the embryo, and puts it in contact with the air to arterialise it. 1087. About the eleventh day the state of the embryo and the parts of the egg surrounding it is as shown in the section fig. 489. 10. Embryo raised upon its side to show the umbilical cord composed of the vitelline vessels and the pedicle of allantois. G68 ANIMAL PHYSICS. 13. Point of suture at the sides of the amnion formed by a fold of the vitelline membrane. 14. The allantois highly developed, having driven back in all directions Fig. 4S7. THEORETICAL SECTION OP THE HEN’S EOG IN THF. FIFTH STAGE OF DEVELOPMENT (Baudrimont and Martin St. Ange). around it the vitelline and chalazean membranes, as well as the membrane of the external albumen so as to apply itself upon the internal surface of the shell, and thus put itself into immediate contact with the porosity of the latter, and to receive the oxygen of the external air and arterialise the blood circulating in its vessels. 15. The cavity of the amnion much enlarged by the accumulation of the amniotic liquid. At the base of the section is seen the point where the junction of the allantois takes place, so as to include in its vascular fold the condensed albumen which remained at the small end of the egg. All the parts of the egg, with the exception of the membranes of the air- chamber and the shell, are thus included on the thirteenth day of the incubation in a close sac. PROGRESSIVE DEVELOPMENT OF THE CHICK. 669 1088. The allantois being developed within the vitelline membrane, it cannot force its way outwards towards the shell, 2 l Fig. 488. THEORETICAL SECTION OF THE llEN’s EOO IN THE SIXTH STAGE OF DEVELOPMENT (Baudrimont and Martin St. Ange). as shown in fig. 489, -without pushing before it the vitelline membrane, as well as the membranes of the ohalazse and the con- densed albumen, which lie outside the latter. This succession of membranes, proceeding from the vitellus outwards, appear overlying each other on the internal surface of the shell in the theoretical section (fig. 489). It would nevertheless be a great error to assume that they actually co-exist as a multiple lining of the shell at any epoch of the development. At the com- mencement of the period of incubation, the vitelline mem- brane is extended over the amnion and the allantois, the 070 ANIMAL PHYSICS. membrane of the chalazae being then superposed upon it, but distinct from it. But at a later epoch, it is impossible to Fig. 4S9. THEORETICAL SECTION OP THE HEN’S EGG IN THE SEVENTH STAGE OP DEVELOP- MENT (Baudrimont aud Martin St. Ange). distinguish them, or rather, the inner membrane alone remains, that of the chalazte having disappeared by atrophy. In fine, membrane after membrane disappears, proceeding thus from without inwards, each inner membrane assuming the function of that which ceases to exist. 1089. Before we resume the exposition of the phenomena of the circulation, it will be useful to reproduce some of the figures drawn by M. Martin Saint Ange, showing with the greatest precision and in their natural dimensions the condition PROGRESSIVE DEVELOPMENT OF THE CHICK. 671 of tlie embryo ancl its accessories at some of the stages of development illus- trated in the preceding theoretical diagrams. The embryo at the mid- dle of the fourth day of incubation is shown in fig. 490. It is supposed to be viewed from the side which corresponds to the yolk, to show the dis- tribution of the blood- vessels and the relation with the allantois. Its state in the middle of the fifth day is shown in fig. 491, the origin of the allantoid vessels ap- Fig. 490. EMBRYO OK THE CHICK OK THE ,FOI BTII incubation (Baudrimout and Martin St. 492. pearing. In all cases the veins are indicated by black, and the arteries by dotted lines. Its state in the middle of the sixth day is shown in fig. In this drawing the embryo is shown in relation to the allantoid and the vitelline vessels, disposed in the form of a lyre. The embryo on the sixth day of incubation is shown in fig. 493. The membrane of the chalazse is here cut to render more distinctly visible the position and relations of the allantois. The embryo on the seventh day of incubation is shown in fig. 494. The relations of the embryo to the yolk and the membranes which envelope it are here shown. day or Aiige). Fig. 491. a. The membrane of the chalaz® disengaged from above the chick. b. The vitellus and vitelliuo membrane which covers the vascular lamina of tho blastoderm. c. The amniotic sac scarcely raised by the liquid it contains. d. The vascular allantois isolated and folded, attached by its pediclo to tho cloaca of the chick. 1090. To resume the exposition of the successive phases of the circulation, we shall take the state of the circulatory apparatus of the chick on the fifteenth day of incubation, when the allantoidal and vitelliuo vascular systems are in a state of nearly equal activity, as is proved by the nearly equal calibre of their principal vessels. G72 ANIMAL PHYSICS. To illustrate this rather complicated system we shall avail ourselves of Fig. 492. EMBRYO OF THE CHICK AFTER FIVE AND A IIAI.F DAYS' INCUBATION (BaudriluOnt and Martin St. Auge). the drawing reproduced in fig. 495-6 from that of M. 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